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Sentry Technical
Industrial 10kA Residual Current Devices (RCDs)
Pulsating d.c. fault current sensitive RCDs
Should a waveform containing a pulsating d.c. component develop
an earth fault, then it is possible that it may not be detected by
an “a.c. only” sensitive RCD. For this reason, the Sentry range
contains RCDs designed to be sensitive to pulsating d.c. fault
currents thus maintaining the intended degree of protection.
Type B RCDs are suitable for situations where there are residual
sinusoidal alternating currents, residual pulsating direct currents
and smooth d.c. and a.c. residual current of various frequencies,
which would not trip Type AC or A RCDs.
These situations can occur in 50Hz a.c. installations with
electronic equipment, e.g. frequency converters, UPS installations,
power supply unit or high-frequency power converters.
The following symbols are used on the front plate of the device to
indicate the type of RCD.
– type AC RCD.
– type A RCD.
– type B RCD.
4. Temperature
All Sentry RCDs are suitable for use in the temperature range
–25°C to +40°C. This is indicated on the RCD by the symbol
-25
.
5. Time Delayed RCDs
S
Type S (or selective)
When two or more Sentry RCDs are installed in series with one
another, measures must be taken to ensure that they discriminate
properly. In event of an earth fault, only the RCD immediately
upstream from the fault should operate.
RCDs do not discriminate on rated tripping current alone, i.e. a
100mA rated RCD situated upstream from a 30mA rated RCD, will
not offer inherent discrimination.
In order to ensure that discrimination is achieved, a Sentry Time
Delayed RCD should be used. The in-built time delay period
ensures that the downstream RCD opens the circuit before the
upstream RCD starts to operate.
The maximum tripping time of a Sentry Time Delayed RCD is
500ms.
Please refer to the current edition of the Wiring Regulations BS
7671 for guidance on the use of these products.
6. 3 phase, 3 wire systems
Sentry 4 pole RCDs may be used to provide earth fault protection
on 3 phase, 3 wire systems, as the current balance mechanism
does not require a neutral to be connected in order to operate
effectively.
Application (continued)