Q & A of the Day – For maximum Zs values, should I use Table 41.3 or Table 41.5?

 

Question: I’ve always completed my test sheets in with either regulations or 80% values earth loop impedance values as per the tables 41.3 and 41.4, or 41.5 

 

I recently tested a factory having a TN-S electrical installation for a friend and the readings were high.

 

An NICEIC qualified manager said that all values can be as per Table 41.5, and he showed me Regulation 411.4.9.

 

I am now confused, as we widely use RCDs more often today. If I can submit 1667 to most circuits, what use are the other tables? I cannot see what purpose they serve, as most items now are RCD-protected. Or was the inspector wrong?

 

For maximum Zs values, should I use Table 41.3 or Table 41.5? 

 

Answer: Where the overcurrent characteristics of an RCBO to BS EN 61009-1 are able to meet the requirements for automatic disconnection, it is quite legitimate to put the figures from Table 41.3 on a certificate or report. In such instances, it is also legitimate to use the figures from Table 41.5 on the certificate or report. 

 

Regulation 411.4.9 states that Table 41.5 may be applied for RCDs, where an RCD is used to satisfy the requirements for automatic disconnection. 

 

Where the Zs value is over the maximum value in Table 41.3, it is logical to use the figures from Table 41.5 on the certificate or report. 

 

The NICEIC advises using Table 41.3 where the installation has been so designed that the overcurrent characteristics of the RCBO to BS EN 61009-1 are being used for automatic disconnection. 

 

A cautionary note must be added here a Zs value that is under 1667Ω only ensures that in the event of an earth fault, the RCD or RCBO should operate. It does not confirm that the circuit is totally compliant with other requirements of BS7671:2008 (2011). For TN-S or TN-C-S systems a Zs value significantly above that required by 41.3 could indicate a high resistance fault somewhere in the circuit which, in turn, could pose a fire risk. 

 

Also, where an RCD is used for automatic disconnection because of high Zs readings, it must be confirmed that the L-N loop impedance is low enough to ensure that the overcurrent protective device operates before thermal damage occurs to the cable.