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12.4. TEST ON DIFFERENTIAL SWITCHES (RCD)
Purpose of the test
Checking that the General (G) and Selective (S) differential protection devices have been
correctly installed and adjusted and that they maintain their characteristics over time. The
check must make sure that the differential switch trips at a current not higher than its
nominal operating current IdN and that the tripping time meets the following conditions,
according to the case:
•
The tripping time does not exceed the maximum time as prescribed by the standard for
differential switches of a General type (according to what described in Table 4
•
The tripping time is between the minimum and the maximum tripping time for
differential switches of a Selective type (according to what described in Table 4
The differential switch test performed with the test key helps so that no “gluing effect”
jeopardizes the operation of the device if it has remained unused for a long time. This test
is only performed to ascertain the mechanical functionality of the device and it is not
sufficient to declare the device
’s conformity to the standard regarding differential current
devices. According to statistics, switch verification through test key, if performed once a
month, reduces to a half the device
’s malfunction rate. However, this test only detects 24%
of the defective differential switches.
Parts of the system to be checked
All differential switches must be tested upon installation. In low-voltage systems, it is
advisable to perform this test, fundamental in order to guarantee a correct safety level. In
medical rooms, this test must be performed periodically on all differential switches as
prescribed by the guidelines.
Allowable values
On each molded-case RCD, two tests must be performed on each differential switch: a
test with a leakage current beginning in phase with the positive half-wave of voltage (0°)
and a test with a leakage current beginning in phase with the negative half-wave of voltage
(180°). The result to be considered is the higher one. The test with ½In must not cause the
differential switch tripping.
RCD type
IdN x 1
IdN x 5 *
Description
General
0.3s
0.04s
Maximum tripping time in seconds
Selective S
0.13s
0.05s
Minimum tripping time in seconds
0.5s
0.15s
Maximum tripping time in seconds
Table 4: Tripping times for general and selective differential switches
Trip-out times
’ compliance with AS/NZS 3017 guideline (**)
½ I
n (*)
I
n
5 x I
n
RCD type
IdN [mA]
t
[ms]
Note
I
≤10
>999ms
40
Maximum tripping time
II
>10
≤ 30
300
40
III
> 30
IV [S]
> 30
500
150
130
50
Minimum non-tripping time
Table 5: Tripping times for general and selective differential switches in AUS/NZ country
(*) Minimum test period for current of ½ I
n, RCD shall not trip
(**) Test current and measurement accuracy correspond to AS/NZS 3017 requirements
