RCD
RCD
A Residual Current Device or RCD is a circuit protective device designed to protect users from electric shock. They are also used in any circumstance where it is not possible to achieve normal operation of traditional protective devices (like Fuses, and MCBs), because the earth loop impedances to high.
What Does it do?
A RCD detects a fault condition which would typically only be seen when a person is receiving an electric shock from the circuit. When this situation is detected it automatically isolates the supply to the circuit (or group of circuits).
This gives greatly enhanced shock protection from both direct contact (i.e. contact with an exposed live wire - say touching a flex you have just damaged with a power tool), and indirect contact (e.g. when the metal casing of an appliance becomes live due to an internal fault) faults. RCD protection is particularly important to protect users in high risk locations where they may be more susceptible to electric shock such as bathrooms, pool areas, saunas, or simply when using power tools or appliances outside (basically anywhere the user can be expected to be wet, barefoot, or in good contact with earth.
What does it not do?
RCDs do not offer any overcurrent protection, and hence they will not clear short circuits or faults that result in an appliance drawing excessive current. When using a RCD protected supply to feed a power tool, they will offer no protection should you make contact with another live non protected circuit with the tool. If you drill into a live cable, the RCD powering the drill will not detect any current flowing from wall cable to drill body to user. Nor can it switch this current off. It thus has no effect on such shock scenarios.
While RCDs act on the majority of shock situations, they won't protect you from all of them (i.e. a shock received by making contact with both live and neutral connections while otherwise being well isolated from earth). Even when a RCD does operate, it can take up to two cycles of the mains (i.e. 40mS or even slightly more for low leakage situations). RCD does not therefore eliminate the risk of shock, rather it reduces it.
How does it work?
RCDs are current balance devices. They measure any current imbalance in the flow in and out of a circuit or appliance via its Live and Neutral conductors (or the combined sum of currents in all phases and neutral on three phase). Should the current imbalance exceed the tripping threshold for the device, it will activate and disconnect the circuit.
For more information see http://en.wikipedia.org/wiki/Residual-current_device
Where are they used?
RCDs are mandated for protection of any circuit that your could reasonably expect to power portable equipment that could be used outside. Hence this would usually include at least the downstairs ring circuits, plus any circuits feeding outbuildings, garages etc. Forthcoming regulations (the 17th edition) will expand this requirement to basically cover all general purpose power circuits unless there is a specific reason not to protect them.
It is not recommended that RCDs be used to protect lighting circuits (except TT installations - see below) since it is known that unexpected loss of lighting as a result of an electrical fault can pose more injury risk than the electrical fault in the first place.
Note that the forthcoming changes to the wiring regulations will make an exception to this rule for bathrooms.
TT Installations
For properties that are not provided with a main earth connection by their electricity supplier (often those supplied via overhead wires), a local earth stake or grid is usually used to provide a local earth connection. Since it is usually not possible to achieve a low enough resistance to earth to allow correct operation of circuit protective devices using this technique, RCD protection is mandated for all circuits. To maintain discrimination between different classes of circuit, and prevent the problems associated with having a "whole house RCD", several RCD devices are typically used. A device with a higher 100mA trip threshold protects all non power circuits, and a lower trip threshold device to protect power circuits.
See Earthing Arrangements for more information.
Types of RCD
There are a number of form factors and technical ratings associated with RCDs. Selecting the correct unit for the job in hand is critical.
Enclosure design
| Type | Description |
|---|---|
| Integrated plug or Socket | There are a range of RCDs that are built into plugs and sockets. These are designed to offer enhanced shock protection to either an individual device or small number of devices. Typically found on extension leads. There are also some fixed wiring sockets that include RCD protection, again designed to provide a safer connection point for certain categories of appliance. |
| Integrated RCD Spur | An RCD integrated into a spur connection unit. Designed to provide individual RCD protection any fixed equipment that has a high electrical shock risk (e.g. a pool/bath hoist for disabled access). |
| Standard DIN rail mounting | This is a modular device in a standard form factor that is designed to be used in electrical enclosures such as consumer units and other similar enclosures. These RCDs typically occupy two module widths (i.e. the space taken by two MCBs), and can be sued to power one or more circuits. |
Electrical and Trip Characteristics
| Type | Description |
|---|---|
| Rated Current | This is the maximum current the device is rated to carry. Commonly used domestic devices in DIN rail mounts are commonly available in 63A and 80A ratings. |
| Number of poles | RCDs are available for protecting both three phase and single phase circuits. (Three phase devices being typically twice the width of single phase ones) |
| Trip threshold or sensitivity | This is the maximum current imbalance that will be tolerated without the trip mechanism being activated. In reality the devices specifications are usually scoped such that the device will trip on 66% of the rated trip current (so as little as 20mA may be required to trip a 30mA device). Common trip thresholds include:
|
| Trip time | General purpose RCDs (sometimes marked with a "G") are designed to trip as soon as possible after a trip condition is detected, and at any rate within two cycles of the mains (40mS for UK 50Hz supplies). There are also time delayed types that are designed to trip only after exposure to a trip fault condition that lasts longer than a pre-set delay (typically two seconds). The time delayed type (of denoted with a "S" suffix) are particularly useful where it is required to cascade RCDs. The time delay maintains discrimination between the cascaded devices so that the once closest to the fault trips first. |
RCBOs
A Residual current Circuit Breaker with Overcurrent protection. These are effectively the combination of a Miniature Circuit Breaker and a RCD in a single unit. Hence they provide RCD functionality and also overcurrent protection. These are very handy devices since they ensure good discrimination should they trip - only the affected circuit is taken out of action. The disadvantage of RCBOs is firstly they are expensive (especially if you need to protect a number of circuits), and secondly many of them are physically larger than a standard MCB. Hence they require the use of a consumer unit with more space.
ELCB
Earth Leakage Circuit Breakers were the forerunner of RCDs. There were two types: the most common was the Voltage Operated ELCB, which detected a large voltage rise on the main earth conductor (which was connected through it). The less common type; the current operated ELCB was in many respects similar to modern RCDs (although the detection method may have worked in a different way).
So when you see ELCB, it could be an old RCD or an even older voltage operated ELCB:
Old voltage operated circuit breaker connected in the meter tails before a consumer unit, with the main earth connection from the consumer unit fed back through the ELCB.
Nuisance trips
A Nuisance trip is and unexpected operation of a RCD that does not appear to be related to an immediately obvious fault. There can be many reasons that these trips occur, some indicate that there is a latent problem with the eletrical installation, some may indicate the presence of a serious but as yet unobserved fault, and others may be the result of a minor fault that in itself poses little if any risk.
Tracing the cause of nuisance tripping can prove to be very difficult and time consuming. This section will attempt to provide some guidelines to help.
What causes nuisance trips?
Excess earth leakage
The RCDs operating principle is to measure the current imbalance between that flowing into and out of a circuit. In an ideal world this difference would be zero, however in the real world there are a various different types of equipment that will legitimately have a small amount of leakage to earth, even operating normally. If the RCD is protecting too many such devices then it is possible that the cumulative result of all these small leakages will be enough to either trip the RCD or pre "sensitise" it.
| Item | Description |
|---|---|
| Electronic equipment mains input filters |
Much modern electronic equipment will include a mains input filter designed to stop electrical noise being passed in or out of the equipment via its mains lead. These typically include a pair of small capacitors connected between the live and earth and neutral and earth wires of the incoming mains lead. The capacitor values will be chosen such that they conduct well at the typical noise frequencies that are intended to be filtered. However a small amount of current flow will occur at mains frequencies, and this results in leakage to earth. It is also worth noting that the filter circuit is designed to snub noise by coupling it to earth. Hence the noise itself can also contribute to the total leakage current seen by the RCD. |
| Heater elements | Many heater elements that are designed to heat water (kettles, immersion heaters in hot water cylinders, or washing machines, ovens, grills etc) use a mineral insulation that is hydroscopic. Hence when left unused for a time they can absorb a small quality of water into the insulation. Since water is electrically conductive this results in a small amount of leakage to the outer (earthed) metal case work of the heater element. Generally this type of leakage poses little if any risk. They way to clear the problem is to run the heater and drive off the moisture. However it is possible to enter a catch 22 situation here, where the RCD prevents the heater from being run. |
| Dampness | An device that handles water and electricity will be vulnerable to dampness getting into electrical connections or wiring harnesses. This can result in short term high levels of leakage that mysteriously vanish later (as the affected item dries out). Even condensation forming in equipment can cause this problem. |
Sensitising RCDs
The effect of high natural leakage currents can be to consume most of the trip current "budget" of the RCD, leaving it very close to the its tripping point. Once this situation has been reached, then even minor changes in circuit environment or use can result in trips. These include:
| Event | Mechanism |
|---|---|
| Switch on surges | When devices with mains input filters are switched on, there will be a brief period where its filter capacitors are "charging up" and passing slightly more leakage than normal. This can be one cause of trips. Also some devices will absorb a large "inrush" of current when first turned on. This can itself generate lots of harmonic noise that is then dissipated to earth by the filter capacitors (same can happen on switch off). |
| Changes in humidity | A simple thing like a damp day can be enough to slightly lower the effectiveness of insulation used in cables and equipment, resulting in more leakage. Electrical installations outside, or in outbuildings are particularly vulnerable to the effects of moisture in general. |
| More appliances in use than normal | Using more appliances than normal or an infrequently experienced combination of them may push the leakage over the limit. |
Wiring faults
| Fault | Mechanism |
|---|---|
| Neutral to Earth shorts | A particularly problematic fault is a short between neutral and earth on a circuit. Since Neutral and earth are nominally going to be at a similar potential (especially in buildings with TN-C-S / PME earthing (see See Earthing Arrangements for more information). You can arrive at a situation where the current flow between neutral and earth is lower than the trip threshold of the RCD. However once the neutral current reaches a high enough level, its potential will be "pulled" away from that of the earth, resulting in increased leakage current flow. Needless to say this threshold will often be reached during transient current peaks caused by equipment being switch on or off. |
| Insulation breakdown or damage | As cables and wires age, their insulation can become less effective. This is especially true if you live in an old property that still has rubber insulated cables. Humidity will also reduce insulation effectiveness. |
Faulty RCD
One obvious possibility (and often overlooked) is that the RCD itself is actually faulty and not tripping at the correct current. A RCD that refuses to reset even when all output connections are removed is an obvious candidate for landfill! Swapping the device with a known good one, or using a RCD test facility as described elsewhere on this page are other ways of finding faulty RCDs. Many RCDs include a "test" button that verifies the unit functions. Note however that this does not test if the trip threshold has drifted too low for example - only that the trip detection and basic mechanics still work.
How to locate the cause of nuisance trips
Empirical tests
There are a number of empirical tests or experiments that you can try to narrow down the source of the problem. We cover some here. The first job is to identify which circuits the RCD is protecting. There is no need to concentrate efforts on examining circuits that are not connected and hence can not be affecting the outcome!
| Do what | Why | ||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Turn off circuits in turn | You may be able to identify which circuit is causing the problem by isolating circuits in turn, and seeing if that prevent the trip from reoccurring. | ||||||||||||||||||
| Remove appliances from suspect circuits | Disconnecting appliances from suspect circuits can let you identify if the fault is in an appliance (the most common situation) or the circuits fixed wiring). If you still get trips with everything disconnected then you may have a wiring fault. If it looks like appliances are to blame, you can apply the "binary chop" principle to narrowing down the field quickly - i.e. unplug half of them and see what happens. If it still trips you know in which half the dodgy appliance probably is. The carry on in the same way - halving the list of remaining suspects, until you get close to the answer. | ||||||||||||||||||
| Check the likely culprits | Identifying which appliances you have from the "high risk" categories listed above can help to take you to the cause of the trouble faster. | ||||||||||||||||||
| Identify coincidental factors | Check for any patterns and relationships between trips and other events. Do they occur only in damp weather, or only at certain times of day, or only when the freezer switches on, or the central heating. Pay particular attention to automated systems (timers, thermostats etc) that can be controlling significant bits of electrical equipment in your home without your manual intervention. | ||||||||||||||||||
| Introduce extra leakage | To correctly test the function and trip threshold of a RCD you need a specialist test meter (see section later on testing). However there are various home built devices that can help you to perform some simple tests. One of these is a leakage plug, which can aid finding problem circuits by letting you introduce a known amount of leakage into a circuit.
To make one you need a conventional 13A plug, fused with a 3A fuse, and and internal connection between earth and live made via a high value high power wire wound resistor. There are no other connections made to the plug externally, and the plug should be clearly labelled. Turning on each of the protected circuits one at a time, and using the leakage plug on it can help identify a circuit that leaks more than the others (since the combination of the plug and its leakage will trip the RCD on the high leakage circuit). A set of plugs wired for different leakages will help you get an approximate idea of the leakage caused by the circuit (and its appliances) itself:
(Don't leave the plug connected for too long, or its resistor will get hot!) |
Measurement Tests
This section deals with the particularly tricky problem of tracking down the causes of nuisance tripping that you have not been able to find by other methods.
For detailed tests on RCDs a specialist test meter is required such as this. An insulation resistance tester such as this may also be required to track down some of the more difficult to locate wiring faults.
However it is possible to carry out a good number of tests using more basic equipment such as a multimeter.
| Warning
There are procedures that are described here that require opening your consumer unit and making temporary wiring alterations within it. If you are not totally confident doing this, then please consult a technically skilled electrician. |
DC Resistance tests
First ensure that power is switched off at the main switch. Ensure all appliances are disconnected from the circuit. These tests require that you disconnect the circuit under test from the consumer unit. In the case of a ring circuit remember to disconnect both legs of the ring. All tests are initially performed on the disconnected ends of the circuit.
There are a number of basic tests that you can do that will identify a great many of the fixed wiring faults that can cause nuisance tripping.
| Test | Purpose |
|---|---|
| Live Neutral Resistance | The first test is a simple resistance test between live and neutral. This test should be done using the highest resistance range on your multimeter. Normally with all the appliances disconnected you would expect to see an open circuit between live and neutral. If this is not the case then you either have something still connected, or you have a serious insulation resistance problem. |
| Live Earth Resistance | This test should also indicate an open circuit with the meter on its highest resistance measuring range. Any non infinite reading here could be a direct indication of your problem. If you get a non infinite resistance reading, you may be able to track down the location of the fault by breaking the circuit up at strategic points (typically by disconnecting part of it at an accessory position). |
| Live Neutral Resistance | Again this test ought to indicate infinite resistance. However it is possible that a very low resistance measurement could exist and yet the circuit still work some of the time (especially on systems with TN-C-S earthing). Unlike a low resistance reading on a Live to Earth test, this fault would not immediately trip a MCB or blow a fuse.
Tracing the location of the short or bridge can again be done using the segmenting procedure described above, and also by careful low resistance measurements made in conjunction with expected cable resistances as found in a Wire resistance table or see Table 9A in the IEE Wiring Regulations On Site Guide. A typical cause of this type of fault, is where a concealed cable has been damaged by a fastening being driven through it. (so if any shelves or pictures have been hung recently, there is a good place to start). |
Insulation resistance
If the DC resistance tests above fail to identify the cause of a circuit that is causing RCD tripping on its own (i.e. without the aid of the appliances usually connected to it. You may find that repeating the tests described using an insulation resistance tester will yield more information. Since the insulation resistance tester carries out the tests at much higher voltages than the multimeter (typically 500V) it will identify those few failures where the conduction path between a live conductor and earth is only visible at mains voltages.
Take care when performing these tests, it is possible to get a nasty shock off an insulation test meter!
Series earth current measurements
A test technique that can be quite handy for testing individual appliances, is to measure the actual current flow in its earth wire connection. To do this safely one needs to make up a suitable teat lead to allow safe access to the earth connection. The best way to measure the current flowing in the earth wire is using a very high sensitivity clamp meter (see next section), since this leave the earth wire connected directly. If one of these is not available, then it is possible to make a measurement using and AC current measurement range on a multimeter. The meter will have to be placed in series with the earth wire and any leakage current will pass through the meter and can be measured. It is important to note however that this carries a certain amount of risk since should there be (or occur) a fault in the equipment, the full fault current may try to pass through the meter. Most decent quality meters will respond by blowing an internal fuse, but some might melt, catch fire, or explode! If the meter fuse does blow it will have in effect opened the connection between the appliance and earth. This will mean its exposed metalwork may now be sat at mains voltage with no protective measure to cause a fuse or MCB to operate.
For these reasons it is not advisable to use this technique at the consumer unit for a whole circuit unless clamp meter is available.
High sensitivity clamp meters
Earth leakage clamp meters like this have recently become a popular way to detect earth leakage faults. These can be safely connected around either an earth wire to directly measure leakage to the equipment earth, or around live and neutral to directly read current imbalance. Since they require no physical connection to the appliance or circuit under test they are a much safer way to measure either leakage to earth or imbalance in the supply to a circuit or and appliance. Measuring the imbalance will help detect where the leakage is not occurring through the circuit or appliances own CPC but instead is finding an alternate path.
Mitigating the effects of nuisance trips
While it is possible to eliminate most causes of nuisance trips with careful system design and testing, it is always wise to design the system to allow for the possibility of it happening:
- Provide dedicated non RCD protected circuits for vulnerable equipment such as:
- Freezers
- Central Heating Systems
- Heated Aquariums
- Fire or smoke alarms
- Security systems and lighting
- Computer and IT equipment
- Have as few circuits or devices as possible protected by the same RCD so that a trip impacts as few extraneous circuits as possible. The ultimate solution would use RCBOs for each circuit. Obviously expense has to be weighed against the implications of tripping.
- Use emergency lighting to backup any important lighting circuits that need to be RCD protected (i.e. on TT earthing systems). In particular these should include lighting for:
- Stairs
- Fire escape routes
- Near trip hazards or other difficult to navigate areas
- Near the consumer unit
- Consider using interruptible power supplies (UPS) to maintain running of critical equipment.
- Power failure alarms might also be an appropriate measure in some circumstances.
System design using RCDs
Some of the system design aspects of using RCDs to good effect is covered in the mitigation section above. However the following basic principles can be applied:
- Use split load consumer units, to allow circuits that do not benefit from RCD protection to be powered directly.
- Don't place too many circuits on the same RCD. In particular identify circuits that are likely to have high leakage (e.g. those containing lots of IT other electronic equipment).
- Where RCDs need to be cascaded, use time delayed types for the upstream device so that trips are contained close to the cause of the fault.
- Don't place circuits to outside electrics and outbuildings on the same RCD as protects the house circuits.
- Avoid placing high leakage devices on RCD protected circuits where possible.
- Ensure accessories and wiring are not placed in excessively damp environments.
- Don't use lower trip threshold devices that is appropriate for the level of risk present and protection sought.