RCD

Work in progress

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 as a circuit protective device where it is not possible to achieve normal operation of traditional protective devices (Fuses, MCBs), due to earth loop impedances to high to ensure correct operation of the fuse or MCB.

What Does it do?

A RCD detects a fault condition which would be typically only be seen on a live circuit, 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 for 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.

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: 10mA 30mA 100mA 300mA
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.

System design using RCDs

TBD

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.

Appliances that typically exhibit high leakage currents

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:

Events that can trip a sensitised RCD

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.

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!

Techniques to try

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	You may find that building yourself a small leakage plug can aid finding problem circuits. 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, will introduce known amount of extra leakage into a circuit. 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: Resistor Vs Leakage Table Resistor Value (ohms) (nearest prefered value) Approx leakage caused (mA) Power rating of resistor required (W) 47K 5 2 22K 10 3 15K 15 4 12K 20 5 10K 25 6 (Don't leave the plug connected for too long, or its resistor will get hot!)

Measurement Tests

(loads more still to do, and so to bed)

DC Resistance Insulation resistance High sensitivity clamp meters series earth current measurements

Mitigating the effects of nuisance trips

There are a number of reasons why these trips occur, and the problem can not be entirely eliminated. Certain strategies are therefore wise to reduce both the occurrence and the consequences of nuisance trips. These are described in the Rewiring Tips article in the following sections:

• Split Load CU
• RCDs
• Dedicated freezer feed
• Stair lighting

RCDs and drills

DIYers sometimes put an RCD plug onto their mains drill in the belief that it will stop them being electrocuted if they drill into a cable. In fact it offers no such protection, and tends to encourage less care, resulting in greater danger.

An RCD on the drill plug does 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.

An RCD plug on a drill can increase risk more than it reduces it by providing a false sense of security.

RCDs and Lighting

Lighting circuits should not be on an RCD in most cases (With the exception of TT installs and bathroom lights, where there are reasons why they are). The main reasons for this are:

1. Lighting is important in a house fire to enable escape, and RCDed lighting is likely to trip during fire conditions, preventing people escaping alive.
2. It is not desirable for lighting to be lost when a fault or shock occurs on socket circuits. This may in fact be a situation in which light is needed urgently.
3. Lighting circuits do not electrocute anyone in the UK, thus are not an electrocution risk to any significant extent.
4. If lights were on a shared RCD, nuisance trips, which do occur with RCDs, would cause sudden loss of stair lighting, and stair falls kill and seriously injure many every year.

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 main earth conductor (which was connected through it). The less common type; the current operated ELCB was in many respects similar to modern RCDs.

So an ELCB could be an old RCD or an even older voltage operated ELCB.

See also

ELCB

My RCD Trips

Wikipedia