JUPITER
IT - 38
9. THEORETICAL APPENDIXES
9.1. TEST ON DIFFERENTIAL SWITCHES (RCD)
Purpose of the test
Checking standards that the general differential protection devices (G) have been correctly
installed and adjusted and that they maintain their characteristics over time. This check
must make sure that the RCD trips at a current non higher than its rated operating current
IdN and that the tripping time does not exceed the maximum time indicated in the
standard for general RCDs (according to the description in Table 2).
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 every six months on all differential
switches as prescribed by standards.
Allowable values
Two tests must be performed on each moulded-case type RCD: 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 ½IdN must not cause the tripping of
the differential switch.
RCD type IdN x 1 IdN x 5 Description
General
0.3s 0.04s Maximum tripping time in seconds
Table 2: Tripping times for general moulded-case type RCDs
Measurement of tripping current for protection differential switches
The purpose of this measurement id to check the real tripping current of general
differential switches (does not apply to selective differential switches).
For differential switches with fixed differential current, this test may be performed in
order to detect possible leakages of the users connected to the system.
Should an earth system not be available, perform the test by connecting the instrument
to a terminal on a conductor downstream of the differential device and a terminal on the
other conductor upstream of the device.
Tripping current must be in the range between ½IN and IN.
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