rcd discrimination

[Daniel Son]

hi guys im an apprentice and need some more understanding about rcds
please may i have an example of rcd discrimination and what ratings i would use for different things and why
thanks danny

 

[MarkieSparkie]

RCD Discrimination:
The old "Rule of Thumb" (RoT) was that the upstream RCD nominal trip rating should be at least 3 times larger than the downstream RCD.
Although this often provides discrimination it is not guaranteed. It was the RoT that was universally used by Sparks at a time when, although time delay RCDs where in manufacturers catalogues, they were expensive and only available by special order, so were rarely used and then usually only in industrial installations.
From the 16th Edition of the Wiring Regulations time delay (S type) RCDs have become increasingly common in domestic installations and today their wider usage has reduced the price close to parity with normal RCDs.
The current Rule of Thumb is:- The upstream RCD should be a time delay (S type) RCD and it's nominal trip rating should be at least 3 times larger than the downstream normal type RCD.

 

[Daniel Son]

RCD Discrimination:
The old "Rule of Thumb" (RoT) was that the downstream RCD nominal trip rating should be 3 times larger than the upstream RCD. Although this often provides discrimination it is not guaranteed. It was the RoT that was universally used by Sparks at a time when, although time delay RCDs where in manufacturers catalogues, they were expensive and only available by special order from the manufacturer, so were rarely used and usually only in industrial installations. From the 16th Edition of the Wiring Regulations time delay (S type) RCDs have become increasingly common in domestic installations and today their wider usage has reduced the price close to parity with normal RCDs.
The current Rule of Thumb is the downstream device should be a time delay (S type) RCD and it's nominal trip rating should be 3 times larger than the upstream normal type RCD.

what rcd ratings should i use and were?

 

[Rockingit]

Markie, unless I need another coffee, isn't that UP & DOWN stream the wrong way around?

 

[Engineer54]

Markie, unless I need another coffee, isn't that UP & DOWN stream the wrong way around?

Yes .... you have it the wrong way round Markie!! lol!!

 

[MarkieSparkie]

Oops! sorry guys ... I rushed it and didn't proof read it before posting ... now edited. Thanks for the heads-up, nice that you guys were on the ball.

A common example Daniel, Upstream device 100mA S type RCD ... downstream device 30mA RCD.

 

[Deleted member 19807]

you need to think about the fault current you are trying to deal with, a normal fault to earth will be in the 10's if not 100's of amps so all rcd's will trip unless time delayed,

I belive there are adjustable delay rcd's available but i have never used them.

have a read of this

http://www.gil-lec.co.uk/custom/upload/pdfs/circuit protection/Hager_residual_current_devices.pdf