Earth leakage, grading and touch voltages

[AusElecEng]

Hi Everyone,

Hoping to get some insight to a problem I'm currently facing with earth leakage protection:

I've been asked to set guidelines for hire generator sets coming onto our site for earth leakage protection. These gensets typically will be connected to boards with final sub-circuits having Residual Current Devices (RCD's) operating at 30mA instantaneously.

My question relates to grading between the RCD's and the upstream Earth Leakage relay on the generator, particularly with reference to step/touch voltage requirements. Our Australian standards reference "IEC61200-413 - protection against contact by automatic disconnection of supply" (which is withdrawn) however in our 415V MEN (TN) system this gives us 60ms disconnection time based on the expected touch voltages - which of course won't grade with anything downstream.

Any insights on how you have traditionally graded earth leakage/RCD would be greatly appreciated.

Thanks,

 

[ScottyUK]

In the British wiring regulations BS 7671 it is permissible to use time-delayed RCDs in applications where they are not provided for personnel protection, e.g. when they are used because of high earth loop impedance, for protection against fire, and so on.

You're unlikely to ever make a time-delayed RCD or RCBO grade with an overcurrent device like a fuse or MCB when responding to an earth fault, but you can get the upstream and downstream earth fault elements to discriminate when both are RCDs. Schneider have a fairly decent selection now in their Acti 9 range - higher tripping currents and longer time delays than in the previous Multi 9 range.