Our Voltimum Experts answer your questions on a daily basis in our Technical Expertise area. This one, about periodic testing and a prospective short circuit test reading, is answered by the NICEIC:
In this installation, there are three flats; one has a CCU with a 30mA main switch. The client doesn't want to upgrade, so can I fit a standalone 100mA time-delayed RCD to provide earth to that flat, and then issue a letter to the client about the main switch not complying.
Answer: IPF is measured normally in Kilo-Amperes (kA), but - in a TT system with high resistance - more realistically Amperes (A). For an example, of the expected result for single-phase system: Ipf = 230V divided by RA.
The protective measure ADS for a TT system, as for the other types of electrical system, limits the magnitude and duration of voltages between simultaneously accessible exposed-conductive-parts and extraneous-conductive-parts occurring anywhere in an installation during an earth fault. ADS achieves this by coordinating the characteristics of the protective device for automatic disconnection, the earthing arrangements for the installation and the relevant impedances of the circuit concerned.
Regulation 411.5.3 requires that where an RCD is used for automatic disconnection as part of ADS, the following conditions are to be fulfilled:
The disconnection time shall be that required by Regulation 411.3.2.2 or 411.3.2.4, and RA x I∆n = 50V
where:
RA is the sum of the resistance of the earth electrode and the protective conductor connecting it to the exposed conductive-part (in ohms).
I?n is the rated residual operating current of the RCD.
Note: Where RA is not known, it may be replaced by Zs.
The requirements of Regulation 411.5.3 are met if the earth loop impedance Zs of the final circuit protected by the RCD meets the requirements of Table 41.5 of BS 7671.
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