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Calculating maximum Za for Time Delay RCDs

fiftyhertz
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

4557a7ce2499a05c386ab10a81e7c400-huge-image.png

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
    Although, the 300ma rcd is not 3x more than the upfront 500ma rcd. What are your thoughts on this?

    There's a small risk of lack of discrimination there - since RCDs usually can trip anywhere between >0.5x and ≤1x their rating, so the 500mA unit (if enthusiastic) could trip with residual currents above 250mA where the 300mA unit (if lazy) might not trip until 300mA - so if you had a residual current in the 250-300mA range (say due to moisture getting in somewhere or cumulative protective conductor currents if you have multiple downstream RCDs) there's a possibility of the 500mA unit tripping rather than the 300mA one.


       - Andy.
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  • 0
    Although, the 300ma rcd is not 3x more than the upfront 500ma rcd. What are your thoughts on this?

    There's a small risk of lack of discrimination there - since RCDs usually can trip anywhere between >0.5x and ≤1x their rating, so the 500mA unit (if enthusiastic) could trip with residual currents above 250mA where the 300mA unit (if lazy) might not trip until 300mA - so if you had a residual current in the 250-300mA range (say due to moisture getting in somewhere or cumulative protective conductor currents if you have multiple downstream RCDs) there's a possibility of the 500mA unit tripping rather than the 300mA one.


       - Andy.
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