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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
    Andy,


    That’s a great explanation and I think ive got it. 


    Say my earth electrode was 20 ohms.


    My upfront adjustable RCD is rated at 500ma, with a time delay of 500ms. Although, I may be able to move this back to 1s due to being a TT sub main, but I may be hesitant to do this as the delay is 1 sec and I’d assume there’d be an inherent trip time of the rcd taking it above 1s.


    My intermediate RCD is an S type 300ma. My electrode resistance is low enough to achieve the 5x mA figure in the table above and therefore trip in 250ms.


    For final circuits I’ve got a 30ma which will trip in 40ms.


    I should therefore have a discriminating system. Although, the 300ma rcd is not 3x more than the upfront 500ma rcd. What are your thoughts on this?
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  • 0
    Andy,


    That’s a great explanation and I think ive got it. 


    Say my earth electrode was 20 ohms.


    My upfront adjustable RCD is rated at 500ma, with a time delay of 500ms. Although, I may be able to move this back to 1s due to being a TT sub main, but I may be hesitant to do this as the delay is 1 sec and I’d assume there’d be an inherent trip time of the rcd taking it above 1s.


    My intermediate RCD is an S type 300ma. My electrode resistance is low enough to achieve the 5x mA figure in the table above and therefore trip in 250ms.


    For final circuits I’ve got a 30ma which will trip in 40ms.


    I should therefore have a discriminating system. Although, the 300ma rcd is not 3x more than the upfront 500ma rcd. What are your thoughts on this?
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