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Calculating maximum Za for Time Delay RCDs

fiftyhertz over 4 years ago

Hi All,

Has anyone any experience calculating time delay maximum earth electrode resistance for time delayed RCDs, im looking at Table 3A in Appendix 3 but cant quite my head around it.

Standard 500ma would get 100ohms Ra.

(50v / 500ma) x 1000 = 100RA

The manufactuer used has setting for the 500ma time delay for 0.15s, 0.25s, 0.5s, 1s, 2.5s and 5s. I cant quite work out how that relates to the attached table.

Are the 0.15s and 0.25s faster than non delay? Id assume not, as its a delay

Some worked examples for the 500ma would be very helpful, im trying to discriminate with a downstream 300ma standard RCCB.

Thanks!

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0 fiftyhertz over 4 years ago

Thanks for all the help so far Andy.

Last question, what are your thoughts on going above 500ma to avoid your example above. Schneider show an example going to 1a in their guide design efficient earth fault protection with RCDs, however I cannot see anything in BS7671 above 500ma, this is only for a sub main and the 50v would still be adhered too?

thanks again
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0 fiftyhertz over 4 years ago

Thanks for all the help so far Andy.

Last question, what are your thoughts on going above 500ma to avoid your example above. Schneider show an example going to 1a in their guide design efficient earth fault protection with RCDs, however I cannot see anything in BS7671 above 500ma, this is only for a sub main and the 50v would still be adhered too?

thanks again
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