What should I record in the disconnection times and max Zs values for a 60898 MCB with no RCD protection?

Question:

When completing an EICR on a TT system what should I record in the disconnection times and max Zs values for a 60898 MCB with no RCD protection? I have been putting 0.2 seconds for disconnection time and keeping max Zs the same as per TN systems but a few people I work with have been putting 200ohms which is maximum recommended for TT? I've always put the max Zs of the breaker/RCBO but just wanted to make sure I am correct?

Thanks
Sam

Answer:

In relation to your question, 200 ohms is the recommened maximum value for the external earth fault loop impedance in a system with TT earthing arrangements. For the maximum disconnection times in a TT  system, you should refer to regulation 411.3.2.4  which will then refer you via 411.3.2.2(final circuits) to Table 41.1.  The maximum earth fault loop impedances for BS EN 60898 circuit breakers are found in table 41.3 and in the manufacturer of the circuit breakers data. However, it is highly unlikely in a TT system that these protective devices will provide adequate fault protection, so the provision of RCD protection would need to be provided. Table 41.5  provides  the maximum earth fault loop impedance values in relation to this.

The above, should provide you with the information required to complete the relevant columns contained in the SOCD, SOTR and code and record appropriately in the EICR if non compliant.

Bob Cairney, Stuart McKelvie, Keith Hagan, Neil Sim and Mark McLaughlin

Experts on all aspects of Electrical Installation Scottish Building Standards

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