8.6.3 - Testing residual current devices (RCDs)

Residual current devices should comply with BS 4293 and are described in (5.9), from which it will be seen that they are provided with a built-in self test system which is intended to be operated regularly by the user. BS 7671 requires that correct operation of this test facility should be checked, and that other tests are also carried out. The time taken for the device to operate must be measured, so the old type of 'go, no-go' tester is no longer adequate. {8.7.1} gives test instrument requirements.

RCD tests are carried out with a special tester which is connected between phase and protective conductors on the load side of the RCD after disconnecting the load {Fig 8.19}. A precisely measured current for a carefully timed period is drawn from the phase and returns via the earth, thus tripping the device. The tester measures and displays the exact time taken for the circuit to be opened. This time is very short, in most cases being between 10 and 20 ms, although it can be much longer, especially for S-types which have delayed operation.

Fig 8.19 - Connections for an RCD tester

1. - General purpose non-delayed RCDs
This is a general purpose type of RCD which is intended to operate very quickly at its rated current. Three tests are required:

a) - 50% of the rated tripping current applied for 2 s should not trip the device,

b) - 100% of rated tripping current, which should not he applied for more than 2 s, must cause the device to trip within 200 ms (0.2 s), and

c) - where the device is intended to provide supplementary protection against direct contact, a test current of 150 mA, applied for no more than 50 ms, should cause the device to operate within 40 ms.

2. - Time-delayed RCDs
In {5.9.2} we discussed the need for discrimination between RCDs. This type is deliberately delayed in its operation to make sure that other devices which are connected downstream of it will operate more quickly. A 3:1 discrimination ratio is required between two RCDs which are connected in series, and this must be verified before testing. It means that the delayed RCD must have an operating current at least three times that of the non-delayed type. For example, to discriminate properly with a 30 mA device, a second connected on the supply side would need to have an operating current of at least 90 mA (in practice, a 100 mA RCD is likely to be used).

The test for the time-delayed RCD consists of applying 100% of the normal rated current, when the device should trip within the time range of:

50% of rated time delay plus 200 ms, and

100% of rated time delay plus 200 ms.

For example, an RCD with a rated tripping time of 300 ms should trip within a time range of:

(150 + 200) ms     = 350 ms and

(300+200) ms       = 500 ms

An RCD tester is an electronic device which draws current from the supply for its operation. This current is usually of the order of a few milliamperes which is taken from the phase and neutral of the supply under test, and will have no effect on the measurement of single-phase systems. However, if a three-wire three-phase system (there is no neutral with this supply) is being tested, the tester must be connected to a neutral conductor to provide the power it needs for operation. Thus, its operating current will flow through a line conductor and return through the neutral, giving a basic imbalance. A 'no-trip' test must also be carried out, during

Fig 8.20 - RCD tester connected for use

which the RCD must not operate when 50% of the rated tripping current is applied for 2 s. The extra current to power the tester, which adds to the test current, may then cause operation. It is necessary in this case to obtain from the RCD manufacturer the value of this current and to take it into account before failing a device on the 50% test.

The RCD tester is connected to the device to be tested by plugging it into a suitable socket outlet (see {Fig 8.20}) or by connecting to phase and neutral with special leads obtainable from the instrument supplier.