Can I calculate how fast an RCD should trip?

spud1 · Mar 9, 2019

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) ?

PEG · Mar 9, 2019

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.....

H · Mar 9, 2019

PEG said:
.....But you calculate your aim point,for a 50 cal,with gravity,wind,earth spin and many other factors,so.....

If you know what your doing.

H · Mar 9, 2019

But I would be very interested if the OP’s quezzie is possible.

M-Ty · Mar 10, 2019

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 · Mar 10, 2019

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.

Deleted member 26818 · Mar 10, 2019

Just use a 1s ‘S’ type 300mA RCD.

static zap · Mar 10, 2019

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

Norah Nabino · Mar 11, 2019

spud1 said:
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