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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!
Parents
  • 0
    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?

    As far as I know there's nothing in BS 7671 to stop you using any rating of RCD you like (provided they comply with the specified product standards). Certainly the tables of helpful information shouldn't be read as limiting your choice to just those values. 1000mA is unusual though - you'd have to ensure that Ra was reliably below 50Ω - which might be a challenge in some circumstances (especially given seasonal variability of soil resistance).


    The other option is to check manufacturer's data carefully. It's quite common to find that manufacturer's data gives much tighter tolerances than the product standards - so it's possible you might find some brand does offer proper discrimination between their 500mA and 300mA devices for example.


      - Andy.
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  • 0
    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?

    As far as I know there's nothing in BS 7671 to stop you using any rating of RCD you like (provided they comply with the specified product standards). Certainly the tables of helpful information shouldn't be read as limiting your choice to just those values. 1000mA is unusual though - you'd have to ensure that Ra was reliably below 50Ω - which might be a challenge in some circumstances (especially given seasonal variability of soil resistance).


    The other option is to check manufacturer's data carefully. It's quite common to find that manufacturer's data gives much tighter tolerances than the product standards - so it's possible you might find some brand does offer proper discrimination between their 500mA and 300mA devices for example.


      - Andy.
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