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8.
Residual Current Device testing
The MFT1800 series can perform the following RCD tests:
1/2I
Non-tripping test at half the rated RCD trip current for 2 seconds, during which the RCD should not trip
I
Tripping test at the rated RCD trip current. The trip time will be displayed
2xI
Tripping test at 2 x the rated RCD trip current (Only available on MFT 1825 and 1835 instruments)
5I
Tripping test at 5 x the rated RCD trip current. The trip time will be displayed in milliseconds.
0 or 180°
Some RCDs are sensitive to the polarity of the supply, i.e whether the test current is applied with the instantaneous
rising or falling. Tests should therefore be performed with the polarity 0° and 180° and the maximum time recorded.
Ramp Test
Used to check the trip current of an RCD.
Fast Ramp test
This comprises a shorter test using fewer current steps compared to the standard ramp test. Allows significantly
more tests in a given time.
The MFT1800 series can test the following RCD types:
AC, A, S, and Programmable (typically a type A RCD with variable disconnection time
The MFT1825 and MFT1835 can also test Type B RCDs.
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 Pulsed
DC and Smooth DC
residual earth currents
Symbol used
also
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 >300ms (>1999ms UK)
No trip
>300ms (>1999ms UK)
No trip
300ms (>1999ms UK) No
trip
>300ms (>1999ms UK)
No trip
1 x I ≤300ms
≤300ms
130ms to 500ms
≤300ms
2 x I ≤150ms
≤150ms
60ms to 200ms
5 x I ≤40ms (30mA RCD’s only)
≤40ms (30mA RCD’s only)
40ms < 150ms (30mA
RCD’s only
40ms < 150ms (30mA
RCD’s only
