Telecare describes a range of products that are designed to monitor vulnerable or otherwise at risk persons to help them live more independently and safely. Where some products are designed to emit local alarms, others are connected to a monitoring service. This connection is often done over the users landline. With the digitisation of the PTSN network by 2027, ISPs and Telecare providers now have the responsibility to ensure that such users still have access to their services when their existing landline goes digital and so require Telecare Battery Backup Solutions.
What’s the problem?
In a PTSN network the telephone line is a copper wire connected directly to the telephone exchange (via a few street cabs). The telephone exchange contains chargers and large battery strings to ensure that even in the event of a utility power failure, that all devices connected to the PTSN network can operate. E.g., if you have a power cut you can still make telephone calls, activate your help cord or personal alarm and get help when you need it.
OFCOM have already provided guidance to VOIP providers that they should provide a 1hr service availability, and the UK government has recently written to a number of providers asking them to consider 8 hour solutions. Consensus appears to be a 4hr solution may be required in future.
Telecare Battery Backup Solutions
The Power Inspired iPowers are DC-DC systems that simply plug in to the DC port on the equipment to be protected. Backup times of one hour can be met with most products whereas the iPower-DC2 is suitable for 4 and 8 hour solutions for systems depending on average power consumption. To extend available backup time for Telecare equipment we suggest powering each separate device from a different iPower.
Another option is to use AC battery backup and protect all Telecare equipment including other essential equipment for in the home. The PF unit can provide pure sinewave output whilst on battery. The large Lithium battery will ensure that even runtimes of 8 hours are easily achievable for constant loads under 100W or so. In addition, the unit can be connected to solar panels prolonging the available runtime or even achieving total grid independence.
Unlike applications where sudden power loss causes data loss or other operational issues, power loss to a pharmacy fridge is not such of an issue since the internal temperature is well controlled. In the event of a power cut a solution is simply not to open the fridge. A typical fridge will maintain the internal temperature for around 4 hours in the event of a power cut – provided the door is unopened. However note if the fridge cannot be opened then no medicine in the fridge can be retrieved.
Many laboratory or pharmacy fridges have alarm contacts which can alert to the fact that power has failed and as a result warn users not to open the door. However, a power fail alarm will have to be operated on a secondary power system, such as a battery, due to the obvious fact that a mains powered system would also be rendered inactive during a power outage. Having a battery system, will also require the battery to be maintained in a state of charge. These added complications mean that such alarms are rarely, if ever, implemented.
A pharmacy fridge will be used to house items, typically vaccines, diluents, immunoglobulins and other medicines with temperature requirements. The costs of these medicines can be quite substantial and if the temperature inside the fridge should rise to over +8°C, then, according to the NHS Green Book, the “cold chain” has been broken and these medicines may need to be destroyed. If not destroyed, then a time-consuming process needs to be instigated to determine the effect on the medicine which most likely will include a reduction in the expiry date.
Clearly, protection against sustained power outages has operational and financial benefits.
Fridge Power Consumption
Instead of giving power ratings of the Pharmacy Fridge, the manufacturers specify the energy consumption in KW for a 24 hour period. The method I found for doing this is here: ENERGY STAR® Program Requirements Product Specification for Laboratory Grade Refrigerators and Freezers, and Ultra-Low Temperature Freezers. This value varies from product to product and depends upon a number of factors, including capacity, the type of doors (glass or solid etc.) and the configuration (bench top, under counter etc.). Typically these figures are around 1KW/24 hour for a typical small system in a typical pharmacy. See Note 1.
The test schedule includes opening the fridge door for a period of 15 seconds (plus an additional 4 seconds for opening and closing), 3 times an hour each hour for 8 consecutive hours. This is useful as it allows us to specify a UPS runtime that will allow a degree of use of the fridge during an extended outage.
A typical fridge compressor has a power draw of around 200W, and will require a sine-wave inverter to ensure correct operation.
UPS Selection
In the table below I’ve created a lookup for the number of hours of runtime you could expect (and remember this includes periodically opening the door) given the energy rating of the pharmacy fridge.
The PF-S-Li products are units ideal for Pharma Fridge applications. The units contain an internal high capacity Lithium Ion battery offering long runtimes, long life and low weight. The PF1200S-Li has a continuous power rating of 1200W, but with a surge rating of 2400W. This allows it to easily deal with the inrush current generated by the compressors of the fridges.
Achievable Runtime in hours:
Energy Rating (KWhr/24hr)
Equivalent Watts
PF1200Li Expected Runtime
0.5
21
>24 hrs
0.75
32
21 hrs
1
42
16 hrs
1.5
63
10 hrs
2
84
8hrs
2.5
105
6hrs
3
125
5hrs
3.5
146
4hrs
4
167
4hrs
4.5
188
3hrs
5
209
3hrs
Contact us to enquire about UPS for Pharma Fridge Applications.
The PF1200S-Li has superb surge rating of twice its capacity for 5 seconds allowing it to cope with the inrush demands of high performance refrigeration units. It also has the benefit of fast recharge and can be connected to a solar panel array. Connectivity is via 4xUK socket outlets and it even boasts a wireless charging pad, USB A and USB C outlets. In addition to powering the fridge it can also provide battery backed power for ancillary devices.
Note 1: I’ve used what manufacturers are displaying on their spec sheets in order to avoid confusion, however the correct term should in fact be kilowatt hours per 24 hour period eg. kWh/24
Power Inspired launch the iPower-DC2 – a DC UPS designed to provide long runtimes on telecommunications equipment. Not only will this keep equipment going in mission critical applications following a power outage – it allows FTTP (Fibre To The Premises) companies to have compliance with OFCOM guidance on providing telephony services for an hour following a power outage. 1
The Power Inspired existing iPower-H is a fantastic solution for this, and indeed is used in many thousands of installations for that very purpose, however with the increased power demands of routers and hubs the need for a more powerful and higher runtime unit became apparent.
Most DC based IT products are 12V, however a proportion are 9V powered and some even 24V. Furthermore, more essential equipment is being powered via USB. In addition several separate boxes can be required in an installation requiring several connections. The iPower-DC2 encompasses all these scenarios with adjustable 9V, 12V or 24V operation, a 2A USB port and 5 DC jack outlets. An adapter can also be used for affixing to a DIN rail on the wall or in a cabinet.
Runtime is impressive with a 10,000mAh Lithium Ion battery pack delivering over an hours runtime at 25W. Full safety is ensured by using UN38.3 certified cells, and with full battery protection circuitry – the battery pack is monitored for overcharge, over-discharge and over-current.
Operation is simple. Set the Voltage Selector switch to the nominal voltage of your power supply and plug in. The iPower-DC2 will start automatically and provide continuous power to the connected loads. If the DC power is unavailable the iPower-DC2 can be cold started – to basically act as a power bank.
To save needless expense and waste, the iPower-DC2 is intended to be used with the AC/DC adapter that comes with the equipment to be protected. However it can be provided with a suitable AC adapter within the same box and any additional leads required. It comes as standard with two 30cm DC-DC leads with sprung connectors suitable for 2.1 or 2.5mm input jacks.
Need any idea how long your UPS will last for? Eg How much runtime will you get out of your UPS? Then this UPS Runtime Calculator is just what you need.
You’ll need to know how much power (in Watts) your UPS is delivering. Then you’ll need to know how many battery blocks and of what Ampere Hour capacity are in your UPS.
This calculator is based upon 12V blocks only and will only accept integer values. So, if you have one single 6V battery of 12Ah capacity, then you’ll need to say it’s a 12V 6Ah battery. If the spec of your battery is not in Ampere Hours but Watt Hours, then as a very rough guide divide the Wh rating by 4 to get the Ah. If you have 7.2Ah or 8.5Ah then if you round down this will give you a minimum, and round up will give you a maximum.
Note, the calculator is approximate. There are no assumptions made on standby current consumption and inverter efficiency. These will be different for different UPS and also different at different load levels. Please just use as a guide. For example if you have an AC load of 1000W, the calculator makes no allowance for DC to AC conversion losses. This allows you to add your own. For example if your system uses 5W in standby, and has an efficiency of 90% then for a 1000W AC load, use 1000 / 0.9 + 5 = 1116W.
If your load varies over time, you’ll need to estimate the average power consumption. You’ll need to size a UPS to meet the maximum power draw expected, but calculate the runtime based upon the average power consumption.
UPS Runtime Calculator
If you want to select a UPS to meet load and runtime calculators please use the UPS Selection Tool.
If you’ve used the UPS Runtime Calculator please leave a comment or drop us a line with any ideas.
Did you know BS7671:2018 Requirements for Electrical Installations, a.k.a. The IET Wiring Regulations 18th Edition states that any socket outlet 32A and under must be protected by a Residual Current Device (RCD)?
Section 4.11.3 is the Requirements for fault protection. Subclause 4.11.3.3 entitled “Additional requirements for socket outlets and for the supply of mobile equipment for use outdoors” states:
In AC systems, additional protection by means of an RCD with a rated residual operating current not exceeding 30mA shall be provided for:
(i) socket-outlets with a rated current not exceeding 32A
BS7671:2018 Section 4.11.3.3
In other words any socket outlet that you plug anything into (basically anything powered from a 13A outlet, or up to 8KVA Systems on Commandos) must have an RCD protecting that circuit. There are exceptions to this, dwellings excepted, but only following a documented risk assessment which clearly states why an RCD would not be necessary.
Purpose of RCDs.
An RCD works differently to a miniature circuit breaker (MCB) or fuse. An MCB renders devices safe in the event of an overload, or short circuit to earth. They are rated in Amps, generally in stages from 1-32A. RCDs work by tripping on an earth leakage fault typically of 30mA. This is a fault current of up to 1000 times smaller than the MCB! RCDs are useful as certain hazards can exist in the event of a fault that will not trip an MCB. Typically this involves applications that are, or may, come into contact with water.
Earth leakage is a small current that stems from phase conductors to earth. This causes an imbalance between live and neutral and it is this imbalance that RCDs detect. If the earth leakage is high enough on an appliance due to a fault or water contact then the equipment chassis can deliver a dangerous “touch current” if a user touches it. The RCD is there to protect against this scenario. If your application has water involved, then it is very difficult for a risk assessment to justify the omission of an RCD from the electrical infrastructure unless other safety measures are taken.
Isolation Transformer
An isolation transformer, by its very nature will stop RCDs from tripping – even in the event of an earth fault. See Isolation Transformers – what you need to know for further reference on this. However this isn’t a problem. In fact, the isolation transformer can make the installation more safe than with the RCD alone. Even a device with a fault can be touched by a user without any hazard occurring. Unless – and I can’t stress this point enough – the isolation transformer has the output Neutral and Earth bonded!
N-E bonds are not there for safety, but rather for noise rejection performance by establishing a zero volt neutral-earth voltage. Isolation transformers in conjunction with UPS Systems provide a very resilient power protection solution. However, in order to ensure the system is safe, then you should not bond the N-E. Our isolated UPS systems leave the system floating, providing true isolation and an inherently safe electrical environment. If you use a N-E bonded system and no risk assessment has been carried out to determine that no RCD is necessary then this contravenes the requirements of BS7671:2018.
Decision Flowchart
Start by asking if there is a documented Risk Assessment as to why there is no need for an RCD on a socket outlet. If there is, then you’re good to go and any UPS is good for this scenario. You can use isolated (floating or N-E bonded) or non-isolated depending upon your requirements.
If there is no risk assessment in place then we check if there is an RCD fitted. If not, or unknown, then in order to provide the safest environment, the solution is a truly floating isolated UPS. Granted, if no RCD is in place, fitting any UPS does not make the situation less safe, it’s just that a floating isolated UPS does make it safe.
If there is an RCD fitted, and no risk assessment has been carried out, then you must not use any NE bonded system NOTE 1. This removes the safety aspect of the RCD.
Conclusion
According to the 2018 Wiring regulations there needs to be an RCD fitted on any sub 32A circuit. This will cause power to be removed if earth leakage of over 30mA is detected. Any standard UPS will not interfere with the operation of the RCD, however an isolated UPS will prevent the RCD from operating.
However, a floating isolated system, where Neutral and Earth are not connected provides a safe electrical environment. In situations where an RCD should be installed, for example there is water required by the application, and the electrical infrastructure is unknown (for example older installations to which RCD was not a mandatory requirement), floating isolated UPS provide the ideal solution.
An isolated UPS that is floating renders RCDs ineffective but provides enhanced safety by removing any touch current hazard.
On the other hand, a N-E bonded UPS system not only negates an RCD but does not make safe any scenario to which the RCD was required to protect against. There’s a reason for section 4.11.3.3 of BS7671 and this situation violates it.
An isolated UPS with a Neutral and Earth Bond renders RCDs ineffective and does not protect against hazards for which the RCD is intended.
NOTE 1: Unless a secondary RCD is fitted to the output of the UPS.
Electricity is mainly generated by turning a large magnet through coils of wire. This induces a clean sinusoidal waveform that can be transmitted down cables, stepped up and down using transformers, to eventually find it’s way into our homes, offices and factories. Along the way, however, some power virusescan interfere with this clean power and cause your equipment power problems. Some problems are obvious, and others not so. There’s generally accepted to be 9 power problems but there’s another problem which is often overlooked and we make it 10.
1. The Blackout
This is one of the most obvious power problems. A complete loss of power. Caused by a variety of reasons, tripped breakers, fuses blown, faults on the utility line, the list goes on. Some power cuts are brief lasting only a moment, for example lightning striking a power line causing protection equipment to operate and then reset. Some may be for hours or days, for example when a cable is dug up by accident. Others last until the breaker is reset. Whatever the cause a sudden loss of power is clearly undesirable for electrical equipment.
Oops!
Only a UPS System can protect against black outs. Your choice of UPS will depend upon the load you are protecting and the amount of time you need support for.
2. The Power Sag
Also known as a power dip, this is where the power momentarily drops. It’s usually caused by the start up of heavy electrical equipment. Other causes include overloads on the network, or utility switching. Note that the plant that is causing the power sag may not be in your building but sharing the same substation. The severity of the dip will impact equipment in different ways. Some equipment will have a natural ability to cope for momentary dips where others will shut down or reset.
You will need a UPS System to protect against a power sag.
3. The Voltage Surge
Some call it a spike, but in any event it’s a short term high voltage on the power line. Usually caused by lightning, which doesn’t have to be a direct hit on the power lines but nearby causing the spike to be induced onto them. The surges are generally destructive in nature as most equipment is not designed to protect against them.
This is where the voltage drops below 10% of the nominal voltage for an extended period of time. This is caused by high demand on the network. The effect is more pronounced the further you are away from the electrical substation. In fact, in rural areas this can be a problem when switching on everyday appliances such as ovens or electric showers. Brown outs affect different equipment in different ways. Computer systems tend to be able to cope well with brown outs as the switch mode power supplies have a wider input voltage. Other equipment that relies on a stable AC source such as lighting, motors or heating will not fare so well. Equipment with linear power supplies such as in high end AV applications may fail entirely.
In order to protect against a brown out you will need some form of voltage regulation. A line interactive UPS System incorporates a boost function to raise the voltage higher by a fixed percentage to bring it into the nominal range. It does this without needing to revert to battery operation.
5. Over Voltage
Also known as a voltage swell this power problem is caused when the demand on the network is lower than normal. This causes the output voltage from the substation to rise. This is a problem when the voltage is over 10% of the nominal. The effects of over voltage can range from overheating, diminished equipment life to complete equipment failure. It’s the inverse of the brown out in that the closer you are to the substation the more pronounced the effect will be.
Similar to the brown out you will need some form of voltage regulation. A line interactive UPS System incorporates a buck function to lower the voltage by a fixed percentage to bring it back into the nominal range.
6. Electrical Noise
This is generally noise between the live and neutral conductors and is called normal mode noise. Its caused by radio frequency interference (RFI) or electro-magnetic interference (EMI). This is usually from electronic devices with high switching speeds. Since the noise carries little energy it generally does not cause damage but rather disruption in the function of other electronic systems. Some filters may remove this, but this is not always effective. The best way to eliminate noise is to recreate the output waveform and this can only be done with an online double conversion UPS System.
7. Frequency Variation
Frequency variation can’t occur on the utility as this would require all the power stations in the country to suddenly change frequency. In fact, the frequency on the national grade is very tightly maintained at 50Hz. However, when you’re not connected to the utility and instead relying on a portable (or even large scale) generator then this can be an issue. As the load increases on the generator and in particular sudden large power draws from them causes the motor to slow down and hence change the output frequency. Some equipment won’t be affected by this at all but it can cause damage to other systems, particularly those with motors or other inductive devices.
More severe than electrical noise, switching transients are very fast high voltage spikes induced onto the power conductors. Caused by the switching off of inductive loads and variable speed drive systems. Such power problems may not be immediately damaging but they can cause degradation of devices subjected to them, particularly if the transient is of high enough voltage.
A surge suppressor can help if the magnitude of the transient is high enough, but these only work at levels above the nominal voltage. This means you could still have a transient of many hundreds of volts entering your equipment. Like with electrical noise a filter will help, but can only reduce a transient not eliminate it. The only way to be sure to eliminate the transient is with the online double conversion UPS System.
9. Harmonic Distortion
Harmonic distortion is where the supply voltage varies from a pure sine wave. The amount of variation is a measurement called the Total Harmonic Distortion or THD. Since we’re talking about voltage we call it THDv, not to be confused with THDi which is a measure of the distortion of input current which is a different thing entirely.
It is generally caused by non-linear loads. These are types of loads that don’t take current in a smooth sinusoidal fashion but instead take it in large chunks. Depending on supply characteristics these chunks of current cause a greater or lesser degree of distortion on the supply voltage. This causes problems for motors and transformers with hum and overheating. In three phase supplies harmonic distortion can actually cause the burning out of neutral conductors and surprise tripping of circuit breakers. Again the only way to eliminate harmonic distortion from your load is to use the online double conversion UPS System.
Summary
That’s the main generally accepted 9 power problems that can cause issues for electrical and electronic equipment. But wait, didn’t I say there was a tenth?
10. Common Mode Noise
This power problem is often overlooked and can cause equipment malfunction. It’s defined as electrical noise between the earth conductor and the live/neutral conductors. Even an online UPS System may not eliminate common mode noise. This is because it is normal practice to have the neutral conductor connected through the UPS from input to output. So although any noise between the live conductor and ground would be taken care of, any noise between neutral and ground is passed straight through to the load.
In a modern electrical infrastructure this generally may not be a problem since the neutral and earth are tied together at the distribution board. This shorts them together and in theory eliminates any voltage or noise between them. However, particularly on long circuits with a lot of equipment on them, voltages can start to be created and common mode noise becomes an issue. Hospital laboratories are a prime example of this.
The way to solve common mode power problems is to isolate the load from the supply. This is exactly what the TX Series does. The in-built isolation transformer creates a new live and neutral, and the online double conversion technology then ensures a high quality stable output. An an added advantage the isolation transformer can provide a safety shield against electric shock which is particularly important in applications where water and electricity may mix. Again, hospital laboratories are a prime candidate. Thus the TX Series can also be defined as Laboratory UPS System. Click for further information on the isolation transformer.
The new summary is this. If you need to provide the highest degrees of power protection against power problems and viruses then the UPS Technology choice should be online double conversion, and the load should be isolated. Choose the TX Series Isolated UPS System.
For the highest degrees of power protection the TX Series of Isolated UPS from 1-10KVA
This article looks at GPON ONT UPS solutions, why you need them and what the solution is. GPON is the acronym for Gigabit Passive Optical Network and is used in “last mile” broadband distribution to provide “Fibre To The Home” or FTTH. Once in your home, the fibre is terminated with an Optical Network Termination device or ONT. If you want to read more about this then try this GPON Fundamentals, but seriously you don’t have to.
What GPON allows is seriously fast broadband into your home but there is a drawback – and it’s nothing to do with broadband, it’s to do with power.
In a typical broadband connection you have copper wires coming into your house. These wires carry a 50V supply which originates from the telephone exchange. This allows the ability to make (and receive) landline calls from your telephone service during a power outage. Essential during any emergency.
GPON however uses fibre optic cables. These cables are made from glass, and glass if you didn’t know is a very bad conductor of electricity (in fact a very good insulator) and so it is impossible to deliver power from the telephone exchange to your home or office. Of course, the result of this is that in the event of a power outage you are unable to make or receive any landline calls.
Depending upon circumstances this may not be too much of an issue. Mobile devices have largely removed the need for landline telephones, however in areas of poor signal quality the need for a landline is paramount.
What’s more, services such as Skype, FaceTime, WhatsApp etc., will also fail as the GPON ONT will be without power and therefore your home or office without internet connection. As well as the fact that you would have to deal with restless children not being able to play on their tablet devices, workers twiddling thumbs etc., there is a more serious note in that you may be completely cut off from any form of communications.
An Uninterruptible Power Supply can provide back up power for just such an eventuality, and for 12V supplied ONTs the iPower is the ideal GPON ONT UPS solution. The iPower has a 12V 2.1A output and in most cases will replace the supplied power supply that came with your ONT. This means that the unit occupies no additional space and simply plugs into your device.
The iPower can provide up to an hour or more back up, depending upon the power requirements of the ONT, enough to protect against the majority of power cuts, or allow you to make an important emergency call in the event of something more serious. For longer runtimes the iPower-HD (coming soon) can provide hours if not days of runtime, or a standard AC system may suffice.
An isolation transformer is a transformer used to transfer electrical power from a source of alternating current power to some equipment or device while isolating the powered device from the power source, usually for safety reasons. Isolation transformers provide galvanic isolation and are used to protect against electric shock, to suppress electrical noise in sensitive devices, or to transfer power between two circuits which must not be connected. A transformer sold for isolation is often built with special insulation between primary and secondary, and is specified to withstand a high voltage between windings.
You probably don’t know it, but your mains supply is most likely provided to you via an isolation transformer. In the electrical substation that feeds your home lurks a huge chunk of copper and iron (the transformer) that takes relatively high voltage electrical power and converts this to our recognised 230-240V voltage that we all know. Your home is supplied with a cable from this transformer that has two conductors. One is the live conductor, and the other is a combined protective earth and neutral (PEN) conductor. (This is known as a TN-C-S system which is the most common in the UK. Other systems are available.)
Once inside your house, the PEN conductor is separated into neutral and earth inside your consumer unit / distribution board aka fuse board. Note that here, the neutral and earth are bonded together which means that the voltage from live to neutral is the same as live to earth – a nominal 230V, and the voltage from neutral to earth is zero (as they are bonded together). Also note that the live conductor via the electricity board fuse, is split into feeds for your different circuits each protected with a circuit breaker or fuse. For extra protection a residual current device (RCD) may also be fitted. Whereas a fuse or circuit breaker will generally require many amps of current to trip or blow an RCD trips with around 30mA of current flow to earth (actually an imbalance between the live and neutral currents which in normal operation are the same). It is used to provide extra protection when contact with water may be experienced, or other potentially hazardous situations. Remember this!
The idea behind this arrangement is for electrical safety. Should a live conductor become detached from inside a piece of equipment and touch the earthed chassis then a high current will flow and blow the fuse or trip the breaker. The same result will be obtained if the equipment should develop a short circuit between live and neutral. If an electric shower has an exposed conductor that water comes in contact with, then there will be a smaller electrical current that will flow from live to earth and this is detected by the RCD which will trip and remove electrical power to the faulty piece of equipment (and everything else on the same circuit). Handy if you’re naked in an earthed bath.
So now we have three conductors at our wall outlet. Assuming we are connected to earth (as we are standing on it), then we will receive an electric shock if we happen to come into contact with the live conductor, but we will be safe if we touch the neutral conductor (as Neutral to earth voltage is zero). If we’re isolated from earth (eg with rubber boots) then we could touch the live conductor and not receive a shock. If we touch both the live and neutral conductors then we will get a shock of course.
The Isolation Transformer for Safety
So how can the isolation transformer be used for electrical safety? It all comes down to what a transformer actually is. In the simplest terms it is two coils of wire around an iron core. The incoming coil – called the primary – converts an electric field into a magnetic one. This magnetic field then induces an electric field on the second coil and hence a voltage appears on the output of this coil (called the secondary). By varying the number of turns in the coils the voltage can be stepped up or down, but in our case the number of turns are equal and so the output voltage is the same as the input voltage. However, the point to grasp here is that there is no electrical connection between the input and the output. The link is done by magnetism. This means that the output is “isolated” from the input and hence the term isolation transformer!
The output of the isolation transformer still has a nominal output voltage of 230V between its output conductors, but there is no link to earth. This means that you can safely touch either conductor without risk of electric shock. You will still get an electric shock if you touch both conductors however!
It is important to note that with an isolation transformer, a device that may have an earth fault that would trip a circuit breaker or blow a fuse will work just fine. In fact, isolation transformers are used for this very reason in certain applications where the sudden disconnection of power due to an earth fault may cause even larger hazards (such as in chemical plants, or in operating theaters). In such cases monitoring is usually provided so that an alarm is raised should this occur.
In the diagram above, taking an installation without an isolation transformer, the device has an earth fault (for example a live conductor has shorted to the chassis). Since Neutral and Earth are bonded in the consumer unit the system sees this as a short circuit and so a large current will flow which will blow the fuse or trip a circuit breaker. It would also trip an RCD if fitted.
When an isolation transformer is put in circuit, nothing will happen. This is because the secondary live and neutral are no longer live and neutral. They really should be called phase 1 and phase 2 hence I’ve put them in quotes. Since they are no longer live and neutral there is no reference to the incoming earth, and therefore no fault current can flow. In this case since there is a fault from “live” to earth, this “live” effectively becomes the equivalent of neutral and the “neutral” effectively becomes live. In the diagram above you would have 230V between “live” and “neutral”, 230V between “neutral” and earth and zero volts between “live” and earth.
However, the main use of an isolation transformer for safety is when people are working live an accidental touch of a live conductor will not cause an electric shock, or that there is risk of damage to cables etc. such as in building sites.
Another consequence of this is that “earth leakage” that is, a trickle of current from live to earth, caused by mains filters, is eliminated. Since there is no direct earth connection, then there is nowhere for the earth leakage to flow to. This can be advantageous in patient vicinity applications or to reduce earth leakage from several devices to avoid nuisance RCD trips.
Use of the Isolation Transformer for reducing electrical noise.
The transformer, being a coil, has what is known as inductance. Inductance is a barrier to high frequency signals. Electrical noise is a high frequency signal and so the transformer acts as a block to this. Other power problems can also be reduced especially if there is an electrostatic screen in the transformer construction which is connected to earth. Any electrical transients between the power conductors and earth can be effectively reduced using this method.
Disturbances between the power conductors can be reduced by the inductance but not eliminated. This is why in dedicated power conditioning devices that incorporate isolation transformers, further filtration is placed on the secondary side of the transformer to reduce this further.
Rather than go into details about this, this piece makes for good bedtime reading.
Or you can just take my word for it.
Redoing the N-E Bond
In complex electrical installations, or some where the wiring may be old, have poor connections or otherwise has excessive impedance, the voltage between neutral and earth can increase, particularly at the furthest points from the distribution board and particularly where high currents are involved. This may, or may not be a problem for your electrical equipment. You could just rebond the neutral to earth again, but electrical codes do not allow for this. However since the secondary is isolated from the primary you can safely derive a new neutral and earth by bonding these together at the secondary of the isolation transformer. This is also done to eliminate noise between “neutral” and earth – as you are shorting it out.
There is a safety concern when doing this however. If, for example, equipment in areas that may come into contact with water (for example laboratories) it is desirous to protect this circuit with a residual current device. This is because water is a pretty poor conductor of electricity and in the event a piece of equipment becoming splashed with water not enough current would flow to blow a fuse, but enough current could flow to give somebody who may be in contact with the water and earth a nasty electric shock. Note that is only takes several milliamps of current to cause heart beat disruption.
Take the scenario above. To protect operators working on equipment with the risk of water contacting live conductors the circuit has been fitted with an RCD. Should water be spilled onto the equipment and come into contact with live conductors a leakage current will flow causing the RCD to operate. This will disconnect power from the equipment and leave the operator safe.
In the next scenario, an isolation transformer has been fitted and supplies the equipment. Should water be spilled now any contact with live conductors will only reference the conductors to earth. No current will flow and hence the operator will be safe and the equipment will continue to operate.
In the final scenario, the isolation transformer has had the earth connected to one of the secondary phases creating a new effective neutral-earth bond. Should water now be spilled on the equipment and come into contact with live conductors a current will flow from the phase end of the transformer, to the equipment, through the water to earth and then back to the transformer. Since this current path is contained within the secondary of the transformer, the RCD will not detect an imbalance and will therefore not trip. The operator is now in an unsafe environment with the potential for an electric shock as they may become the lowest point of resistance for the leakage current.
It is not only water where such hazards can exist. I recall being told of the case of an unfortunate checkout operator at a major grocery store chain. Unbeknown to her, an electrical cable feeding some equipment had become entangled in her chair mechanism. As she swivelled in the chair this caused a cut in the insulation of the cable which then contacted the live conductor. This circuit was not protected by an RCD but only by circuit breakers. It would therefore take a fault like current to trip the breaker. In this instance the chair made a poor connection to earth and so the chair – and the unfortunate operator – were now at live potential. Everytime she touched something that was earthed – such as the till or conveyer mechanism – she received an electric shock. If the circuit was protected with an RCD then this would not prevent an electric shock but the severity would be reduced and it would only happen once, rather than the multiple times it happened to this poor lady until power could be removed. The retrospective action was indeed to fit RCDs (and do this in all stores). If they were to fit an isolation transformer then the operator would not have received an electric shock at all. No fault would be apparent – save for a visual inspection. If they were to fit an isolation transformer with a N-E bond on the secondary, then this would have negated the effect of the RCD rendering another dangerous situation for the operator.
Transformer Regulation
Transformers are not perfect and impedance exists in them that causes a volt drop within the transformer when current flows. The more current that flows the larger the volt drop and so the output voltage falls. The regulation of a transformer is the difference in the no-load voltage to the full load voltage expressed as a percentage. Poor regulation can introduce other problems into a circuit. For example, if the load is non-linear and takes current in high value chunks – such as in rectifiers, then the poor regulation can cause waveform distortion and introduce voltage harmonics into the system. Other problems include the voltage falling too low and causing under-voltage protection systems to operate.
UPS and Isolation Transformers
Before I go into UPS with isolation transformers it’s probably worth mentioning what happens with transformerless UPS Systems in the event of an earth fault like described above. Earth leakage is not eradicated using a UPS. In fact it is cumulative so the earth leakage of the UPS is added to the earth leakage of the connected loads. This is a consideration for pluggable UPS but that is the subject of another article. If an earth leakage event occurs that trips the RCD then power to the UPS will be lost and the UPS will do what it is meant to do and that is continue to provide power to the connected load – even if it does have a fault. Note I’m assuming here that this is a fault in the order of tens of milliamps- enough to trip the RCD but not enough to blow a fuse or trip a circuit breaker. This you would feel is a hazard. However, when a UPS is operating from battery it will have (pluggable systems – not always the case on hardwired systems) a back-feed relay. What this does is open to prevent the output of the inverter being present on the incoming supply pins on the UPS. This is effectively the same as isolation. The load is now isolated from the source and therefore no earth leakage current will continue to flow and therefore no hazard will exist.
When a UPS has an isolation transformer this provides added power protection but it does require certain considerations. Firstly, it requires a big chunk of copper and iron to be added to it, substantially increasing its weight and physical size. As described above, the creation of a neutral-earth bond on the UPS secondary causes any RCD protection to be redundant, so it is preferable to have the transformer floating. On hardwired UPS systems, if a N-E bond is desired this can be added by the site installers quite easily and any RCD protection installed downstream of the UPS. Also, where in the UPS circuit should the transformer go? Should it be on the input or the output?
If it is on the input, then the UPS has the added benefit of the protection afforded by the transformer. It means that the earth leakage of the UPS (and connected equipment) is zero as measured on the input to the UPS.
If it is on the output, then the UPS output will always be consistent whether or not it is running from battery power or in normal operation. This would be especially important if a N-E bond is required.
In my opinion we consider an input transformer to be the best option, coupled with a truly floating output. This is the safest configuration and one we have incorporated into our TX Series UPS systems.
Edit – Floating Voltages
Adding this to the original article to explain in detail why the output voltages to earth are as they are.
If we take our isolation transformer on which the output secondaries are not connected to earth. Try as we might there will always be some parasitic capacitance between the output phases to earth, the impedance of which we will call Zp.
Then we measure, (using a high impedance voltmeter) between Phase 1 and Phase 2 and we will get the output voltage Vo. Now measuring between Phase 1 and Earth, what will we expect to find? We are measuring the voltage across the parasitic impedance Zp. Assuming this is the same between phase 1 and earth as is between phase 2 and earth, then the voltage measured will be Vm = Vo (Zp / (Zp + Zp) ), or Vm = Vo/2, eg what we measure is half the output voltage. So for a 230V transformer we would expect to measure around 115V.
If we connect a piece of equipment to the transformer that contains an input filter, then we will find there are capacitors intentionally connected between the input phases and ground. Ignoring Zp (as Zc≪Zp), then Vm = Vo(Zc/(Zc+Zc)) Eg half Vo again.
This is why the measured voltage between phase and ground tends to be around half the transformer output voltage. I can see why at first glance this may cause concern, as it appears that we have a high voltage to earth even via our isolation transformer. However no current will flow (and hence it is safe) if we make a connection between any phase and earth. All we do is now reference that phase to earth.
