How to test RCD when RCD is RCD protected? | on ElectriciansForums
Click here to remove these ads

How to test RCD when RCD is RCD protected?

Post new thread

Discuss How to test RCD when RCD is RCD protected? in the UK Electrical Forum area at ElectriciansForums.net

Calebp43

Calebp43

Joined
Dec 2, 2021
Messages
72
Reaction score
36
Location
Wales
Got a sub main board with an RCD inside. Need to test that but whenever I test it it sometimes trips the RCD from the main board. I need to test the RCD from the sub main board. Any suggestions?

My thoughts are to bypass the RCD at the main side with connector blocks momentarily but surely there is another way.
 
Sort by date Sort by votes
As above, although you have to ask about the nature of the circuit design and apparent lack of discrimination if this is so. Very few reasons to protect a standard sub-mains feed with RCD, even less if it's not a cascading system of design with variations on trip time.
 
Upvote 0 Downvote
Calebp43 said:
Got a sub main board with an RCD inside. Need to test that but whenever I test it it sometimes trips the RCD from the main board. I need to test the RCD from the sub main board. Any suggestions?

My thoughts are to bypass the RCD at the main side with connector blocks momentarily but surely there is another way.
Click to expand...
I'm not sure why that's a problem. If the downstream RCD is tripping the upstream RCD then the downstream RCD must be working. As is the upstream one. Right?
 
Upvote 0 Downvote
Rockingit said:
As above, although you have to ask about the nature of the circuit design and apparent lack of discrimination if this is so. Very few reasons to protect a standard sub-mains feed with RCD, even less if it's not a cascading system of design with variations on trip time.
Click to expand...
It's all the rage in the air con industry.
[ElectriciansForums.net] How to test RCD when RCD is RCD protected?
 
Upvote 0 Downvote
Ray Pooley said:
I'm not sure why that's a problem. If the downstream RCD is tripping the upstream RCD then the downstream RCD must be working. As is the upstream one. Right?
Click to expand...
Errmmmmm........ Nope. You know how an RCD works and what it does, yes?
 
Upvote 0 Downvote
Ray Pooley said:
Yes I know what an RCD does. If you have two in series and you trip #2 you're going to trip #1 as well.
You don't think so?
Click to expand...
I don't think so, I know so. And, I'd question your knowledge of RCD's.

Hypothetically.... two commonly found 30mA AC type RCDS, in series, feeding a fixed load (with no other overload protection for the sake of making this simple...). 230v Circuit has a Zs of 0.50Ω therefore a PFC of 460A. So an earth fault occurs, 400+ Amps flows instantaneously. Which RCD goes first?
 
Upvote 0 Downvote
Rockingit said:
I don't think so, I know so. And, I'd question your knowledge of RCD's.

Hypothetically.... two commonly found 30mA AC type RCDS, in series, feeding a fixed load (with no other overload protection for the sake of making this simple...). 230v Circuit has a Zs of 0.50Ω therefore a PFC of 460A. So an earth fault occurs, 400+ Amps flows instantaneously. Which RCD goes first?
Click to expand...

Reminds me of Matt Lucas and David Walliams

That One No!! That one.

​

 
Upvote 0 Downvote
pc1966 said:
The A/C units we got specified 30mA protection, even though fed in SWA, etc. They may not actually need it, but only sane option was to follow the manufacturer's instructions.

Not sure if it stems from EU markets where TT earthing is the norm?
Click to expand...
I found this on my last commercial plant room project - AHP's were all manufacturer specified to have 30mA RCD protection (and 4p as well). I think it's a risk liability thing rather than an engineering issue.
 
Upvote 0 Downvote
Ray Pooley said:
Yes I know what an RCD does. If you have two in series and you trip #2 you're going to trip #1 as well.
You don't think so?
Click to expand...

Let's say you take your MFT and test #1 at it's rated current and it trips in 19ms
You test #2 at it's rated current and it trips in 75ms
(Both times are less than the manufacturing standard requires and both would be deemed to be in fine health)

Earth leakage of exactly the rated current occurs, and RCD #1 duly trips in 19ms. Fairly clearly RCD #2 isn't going to trip, the in-balance will not be present at the coil of RCD #2 by the time it was going to trip.

Other factors include the size of the leakage current and where in the waveform cycle the fault occurs.
For these reasons on a properly designed system each additional RCD's in series needs have 'l delta n' (trip current) at least 3x greater than the previous RCD guarantee selectivity.
 
Upvote 0 Downvote
So I would connect the live to the live outgoing of RCD, the neutral to the neutral outgoing of the RCD and the earth to the neutral incoming of RCD?
pc1966 said:
Connect your MFT earth lead to the neutral feeding the 2nd RCD.

That way it diverts the current around it so it should see the fault and trip, but the up-stream RCD sees nothing unusual
Click to expand...
 
Upvote 0 Downvote
pc1966 said:
The A/C units we got specified 30mA protection, even though fed in SWA, etc. They may not actually need it, but only sane option was to follow the manufacturer's instructions.

Not sure if it stems from EU markets where TT earthing is the norm?
Click to expand...
Somewhere on here I quoted a response I got from Daikin who specified type B RCD protection.

There response was something along the lines of "yeah, that's standard wording o. All quotes but we'll accept installation to local code" 😂

Which was a relief as a three phase type B RCD is upwards of £600, and I needed eight of them...
 
Upvote 0 Downvote
Rockingit said:
As above, although you have to ask about the nature of the circuit design and apparent lack of discrimination if this is so. Very few reasons to protect a standard sub-mains feed with RCD, even less if it's not a cascading system of design with variations on trip time.
Click to expand...
Been thinking a lot about your comment. Shouldn't the cable that's being fed from the main board to the sub main be RCD protected?
 
Upvote 0 Downvote

Reply to How to test RCD when RCD is RCD protected? in the UK Electrical Forum area at ElectriciansForums.net

Similar Threads

F
TT System Existing is a multi9 13 way RCD main switch DB
Regarding the EV, it’s an Ohme charger which I believe has a type A RCD built in, setup would be: 50A RCBO to feed garage db Garage db has no...
Replies
17
Views
529
fattony
F
L
  • Question
UK The RCD test button on my tester trips the wall sockets
My Understanding is the 6 Months interval is IET Guidance and this wording is incorporated onto the label which is a requirement in 514.12.2 where...
Replies
9
Views
402
PS Electrics
PS Electrics
gazdkw82
  • Question
RCD testing issues
Assumption is the mother of all major foul-ups! Start with the basics, like IR testing, RCD testing on its own, and checking the leakage as...
Replies
7
Views
637
pc1966
P
impish15
Delayed tripping on RCD
Ok I’ll try this when I’m down
Replies
4
Views
286
impish15
impish15
J
RCD trips
In my opinion replacing the consumer unit and waiting to see which RCBO trips is not a good fault finding technique. If the lighting circuit...
Replies
8
Views
447
davesparks
D

OFFICIAL SPONSORS

Electrical Goods - Electrical Tools - Brand Names Electrician Courses Green Electrical Goods PCB Way Electrical Goods - Electrical Tools - Brand Names Pushfit Wire Connectors Electric Underfloor Heating Electrician Courses
These Official Forum Sponsors May Provide Discounts to Regular Forum Members - If you would like to sponsor us then CLICK HERE and post a thread with who you are, and we'll send you some stats etc

YOUR Unread Posts

Tilers Community
This website was designed, optimised and is hosted by untold.media Operating under the name Untold Media since 2001.
NHS Forums | Forum for NHS Staff and NHS Patients
Back
Top
✖
AdBlock Detected

We get it, advertisements are annoying!

Sure, ad-blocking software does a great job at blocking ads, but it also blocks useful features of our website. For the best site experience please disable your AdBlocker.

I've Disabled AdBlock    No Thanks