time delay rcds

[Farmelectrics]

were would you use a time delay rcd and what are the benefits of doing so is it just in tt installations

 

[brman]

If you are installing a socket-outlet to be used for mobile equipment outdoors, then the socket-outlet will require 30mA RCD protection.
The problem you have with installing a time delayed up front RCD, is what delay you will use.
1s for distribution circuits, or 0.2s for final circuits?

I don't think I am explaining myself very well

The outside socket is actually on another CU which also supplies some sheds and garages. This already has RCDs so I don't actually have to do anything to meet the regs.

However I spotted that the main house has not got RCDs and am trying to do something about it while I am there.
In this case I would say I am officially protecting a distribution circuit (i.e. the tails) so am ok with 1s but in practice I am also trying to ensure all the final circuits have fault protection which would suggest 0.2s.

This is my dilemma. Taken in isolation perhaps I should use a non time delay to give the 0.2s on all circuits but if someone then wants to add another circuit with an RCBO they will have a discrimination problem.

 

[brman]

perhaps I should just tell him it is a new CU with proper RCD protection or nothing and hope my previous comments about lack of protection scares him into accepting that

 

[Welshsparky]

I have just done one of them on a farm, main incomer had 100ma rcd then several submains protected by a 30ma rcd, problem they had was when their was a trip, for some reason 9 times out of 10 the main rcd would go and they'd have to wander up to the barn to reset it, installed a time delay version and all is now hunky dory!

 

[Adam]

Slightly off topic!

Why do you install 100mA RCD's?
if your Ze is low enough, there is no reason you can't use a 300mA+.

 

[JD6400]

The 16th allowed up to 500mA but that is now down to 300mA .

 

[Taylortwocities]

Thanks Engineer 54 , i had never heard of the x3 recommendation .

That's because for RCDs it doesn't happen. As Eng 54 has said.
I rather think the x3 refers to discrimination of fuses, mcbs etc.

Lets say you have a 300mA RCD main switch(non time delayed) and sockets protected by a 30mA RCD.
Which RCD will trip first if there's a big earth fault on one of the sockets of more than 300mA? Its the RCD with the faster (time) trip characteristic or it may depend on where the AC sine wave is at. It could really be either, or - sometimes - both of them.

An S-type is the only way to ensure discrimination of RCDs.

 

[JD6400]

Thanks Taylortwocites , that would generally be the case as you say . It was the 3x rule of thumb that i have not heard of before , well for RCD's anyway .

 

[Engineer54]

That's because for RCDs it doesn't happen. As Eng 54 has said.
I rather think the x3 refers to discrimination of fuses, mcbs etc.

Lets say you have a 300mA RCD main switch(non time delayed) and sockets protected by a 30mA RCD.
Which RCD will trip first if there's a big earth fault on one of the sockets of more than 300mA? Its the RCD with the faster (time) trip characteristic or it may depend on where the AC sine wave is at. It could really be either, or - sometimes - both of them.

An S-type is the only way to ensure discrimination of RCDs.

No, this was a stated quite clearly in a few of the manufactures catalogs in the past, Merlin Gerin, Siemens being two of them.

Discrimination using this method (without a time delay) say a 100mA upstream of a 30mA, only works with increasing natural leakages, any fault on that downstream circuit that produces a fault leakage current exceeding 100mA, ....and both RCD's are likely to trip simultaneously.

As you say, all RCD's/RCBO's have a pretty wide ranging tripping speed, which can also affect discrimination between series connected RCD devices. But the only way to ''Ensure'' discrimination in all normal circumstances, is by means of an S type device being used upstream...

 

[infinity]

Also there would be no need to use a 100mA rcd main switch if all circuits had 30mA Rcbo's would there?

 

[Engineer54]

Also there would be no need to use a 100mA rcd main switch if all circuits had 30mA Rcbo's would there?

Your thinking too small here. There are far more different distribution layouts than a domestic CU, be they dual RCD or all RCBO layout!!

 

[Taylortwocities]

Also there would be no need to use a 100mA rcd main switch if all circuits had 30mA Rcbo's would there?

But you are correct, for a domestic TT all RCBO there is no need for additional 100mA RCD incomer.
Just a normal DP main switch.

 

[Engineer54]

But you are correct, for a domestic TT all RCBO there is no need for additional 100mA RCD incomer.
Just a normal DP main switch.

That depends, what are you going to do for say a supply going out to an outbuilding or garage with a mini consumer unit protected by it's own 30mA RCD device??

 

[Tonkatoy27]

Time delay RCD doesn't have to cost the earth. I hate to say it but Denmans Control Gear Direct 100mA S type is dirt cheap.

 

[alarm man]

But you are correct, for a domestic TT all RCBO there is no need for additional 100mA RCD incomer.
Just a normal DP main switch.

i thought you needed the 100ma as main switch if the cu was metal.that seems the norm on the ones ive seen

 

[Engineer54]

i thought you needed the 100ma as main switch if the cu was metal.that seems the norm on the ones ive seen

I think your right there, but it would still have to be an S type, if you wanted to maintain discrimination!!