The characteristics of each RCD type is detailed below:
RCD Type AC A S B
Description
Operate with AC
residual earth currents
only.
Operate with AC and
pulsed DC residual
earth currents.
Selective RCD
Operates on type AC
with time delay or
type A with time delay
Operate with AC
Symbol used
S
also
S
Application General purpose
protection of
Sinusoidal AC
supplies.
Protects against
AC and pulsed DC
(rectified AC).
For use upstream
of a standard AC
RCD to prevent trip
contention.
ie. Allows local trip
to operate first.
TIP: Remember “S”
for “Slow tripping”
Special applications
where protection of
DC, as well as AC
earth faults may be
encountered.
Other types will not
operate on DC fault
currents.
Trip times Trip times as defined in BS EN
½ I
>300 ms
(>1999 ms UK) No
trip
>300 ms
(>1999 ms UK)
No trip
300 ms
(>1999 ms UK)
No trip
>300 ms
(>1999 ms UK)
No trip
1 x I ≤300 ms
≤300 ms
130 ms to 500 ms ≤300 ms
2 x I ≤150ms ≤150ms 60ms to 200ms
5 x I
≤40 ms
(30 mA RCD’s only)
≤40 ms
(30 mA RCD’s only)
≤40 ms <150 ms
(30 mA RCD’s only)
≤40 ms <150 ms
(30 mA RCD’s only)
8.1 Making an RCD measurement
NOTE :
To select 0º or 180º press and release the mode button
whilst in RCD test mode
(Note: Type B is not available on MFT1815 instruments)
10 mA and 30 mA RCD’s should be tested at ½ x I, 1 x I and 5 x I
All other RCDs only need to be tested at 1 x I
I = trip current rating of the RCD
2 x I available on the MFT1825 and 1835 only.
Connecting the neutral test lead in either of the above options will not affect the RCD but will detect a reverse
polarity and, the testing will be inhibited.
8.2 Selecting RCD type
Using the right hand (secondary range knob) select the RCD trip rating.
This is printed on the RCD (10 mA, 30 mA 100 mA etc.)
Select the RCD type, either AC, A, S or B by pressing and HOLDING DOWN the mode button
for 2 seconds
whilst in RCD test mode. Repeat until the RCD type is displayed. Refer to table above for symbol options and
descriptions.
NOTE : Type B RCD testing is NOT available on MFT1815.
www.megger.com
MFT1800 series
38
Residual Current Device (RCD) or Ground Fault Current Interrupt (GFCI) testing
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