Discuss Can I calculate how fast an RCD should trip? in the Electrical Forum area at ElectriciansForums.net

spud1

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Arms
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Hi all.

Been bugging me.
I think there is a way to calculate how fast a particularly rated RCD should trip (rather than measuring) but I can't remember how to do it ?
e. g. If I was designing an SWA submain fed from a TT earthed installation and I needed to install say a 300mA TD RCD to acheive a disconnection time of 1sec for fault protection, is there a way to calculate at the design stage whether that RCD will trip within 1sec ( if I know the rated time delay) ?
 
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PEG

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Mentor
Arms
Esteemed
I will stick me neck out,and say no.
You are not only dealing with differing types of electrical initiation,but there are also mechanical factors,such as thermal,friction and spring parts of its action.

The measured values have a time,but the expected,are always more than/less than.

.....But you calculate your aim point,for a 50 cal,with gravity,wind,earth spin and many other factors,so.....
 
I would suggest looking up the data sheet for the RCD in question. You'll see the I/t curves have fairly steep sides which include manufacturing tolerances and the best you can calculate is to ensure your design falls on the "will trip" side.
 

Richard Burns

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Mentor
Arms
Esteemed
As far as I am aware the actual time in which an RCD will trip is determined by the manufacturer of the device.
Different makes of RCD/RCBO will trip at different speeds and even then the same manufacturer might have different variabilities in the speed of tripping.
However for all modern unadjustable RCDs if the fault current is 5 * rated current or more they must, to be compliant, trip within 40ms if not time delayed at least 150ms if time delayed.
In preparing a design you should be able to rely on these values to determine if the installation is safe.
 
Quick enough to save a human (if they need saving !)
, and it's self from
thermal damage.
So this will vary by :-
a) Manufacturer-
b) Bulk of material
c) Quality of materials used
d) May correlate with price/safety margin/cycles good for
 
Last edited:
Hi all.

Been bugging me.
I think there is a way to calculate how fast a particularly rated RCD should trip (rather than measuring) but I can't remember how to do it ?
e. g. If I was designing an SWA submain fed from a TT earthed installation and I needed to install say a 300mA TD RCD to acheive a disconnection time of 1sec for fault protection, is there a way to calculate at the design stage whether that RCD will trip within 1sec ( if I know the rated time delay) ?
Adiabatic equation
 
D

Deleted member 26818

The adiabatic equation would work for an MCB or fuse, not for an RCD.
 
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