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

 

[Des 56]

Agricultural perhaps,where you have "all other circuits" not exceeding 300 m/amp

 

[Adam]

A time delayed RCD should always be used if other RCD's / RCBO's are connected after them.

This will allow the other RCD's to operate before triggering the time delayed RCD.
These are generally used in TT installations as the main switch when 30mA RCD's are also used for socket outlet circuits.

 

[Farmelectrics]

thanks adam thought it was mainly tt systems

 

[Engineer54]

As Adam has stated, S type RCD's will always be required for reliable discrimination with downstream RCD's/RCBO's. The other recommendation is that the upstream RCD device be at least 3 times that of the downstream, well let me tell you now, that rarely if ever works!! Time delay is the ONLY means of achieving discrimination between RCD devices!!

 

[JD6400]

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

 

[brman]

Guys, at the risk of going a bit of topic can I expand the question a bit?

What if you are fitting a 100mA RCD to protect a TT installation where there are no other RCDs currently fitted to any circuits? Would you....
a) use a non time delay RCD because they are cheaper and you don't have any other RCDs to require discrimination or
b) use a time delay so upgrades to circuits in the future can be done by adding 30mA RCDs without causing a discrimination problem

This is actually a situation I am quoting for at the moment. Customer doesn't want the cost of a CU upgrade (which I have recommended to give 30mA on sockets, bathroom etc etc) but currently has no RCDs and a Ze of about 50 ohms which I have told him isn't giving him any fault protection.
btw. for what it is worth I have gone for option b).

 

[Jimmy Boy]

Well use a S Type delayed as the main switch, that's a saving on a main switch, and a 30 Ma RCD on those circuits that require it, which without knowing the job is all of them possibly, I don't want to come across as critical your not insisting on RCD protection on a CU change ? and don't forget also the enclosures type needs to be a class 2 or all insulated for TT's as I interpret the regs
ATB J

 

[brman]

Well use a S Type delayed as the main switch, that's a saving on a main switch, and a 30 Ma RCD on those circuits that require it, which without knowing the job is all of them possibly, I don't want to come across as critical your not insisting on RCD protection on a CU change ? and don't forget also the enclosures type needs to be a class 2 or all insulated for TT's as I interpret the regs
ATB J

I think you misunderstood me. I was not quoting for a CU change. The work asked for was an outside socket off another, RCD protected, CU. I just noticed that the main CU supplying most of the house circuits didn't have any RCD protection and told the guy that wasn't clever. Not suprisingly the guy doesn't want a £100 job turning into a £500 job with CU change so wants the minimum that will give him some protection. That would be an up front 100mA RCD without touching the main CU.

 

[Jimmy Boy]

I think you misunderstood me. I was not quoting for a CU change. The work asked for was an outside socket off another, RCD protected, CU. I just noticed that the main CU supplying most of the house circuits didn't have any RCD protection and told the guy that wasn't clever. Not suprisingly the guy doesn't want a £100 job turning into a £500 job with CU change so wants the minimum that will give him some protection. That would be an up front 100mA RCD without touching the main CU.

Sorry dude got wrong end of stick, just note it on the minor works to cover your ---

ATB J

 

[jamie-spark]

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

agricultural and horticultural
705.442.7

 

[jamie-spark]

Oops did not see Des post on my phone

 

[Jimmy Boy]

agricultural and horticultural
705.442.7

good point but it does then refer you to 411.1 >415.1.1
so round we go in circles lol

J

 

[brman]

Sorry dude got wrong end of stick, just note it on the minor works to cover your ---

ATB J

don't apologise, I didn't exactly make it clear in my first post!

Yep, arse will be covered but it looks like I have persuaded him to have an up front RCD fitted so at least he will get some protection.

 

[Deleted member 26818]

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?

 

[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 .