How to test RCD when RCD is RCD protected?

[Calebp43]

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.

 

[pc1966]

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.

 

[Rockingit]

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.

 

[Ray Pooley]

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.

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?

 

[GBDamo]

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.

It's all the rage in the air con industry.

 

[Rockingit]

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?

Errmmmmm........ Nope. You know how an RCD works and what it does, yes?

 

[Rockingit]

It's all the rage in the air con industry.
View attachment 99515

Muppetry at it's finest.

 

[Ray Pooley]

Errmmmmm........ Nope. You know how an RCD works and what it does, yes?

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?

 

[Rockingit]

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?

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?

 

[westward10]

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?

I assume you understand selectivity.

 

[mainline]

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?

Reminds me of Matt Lucas and David Walliams

That One No!! That one.

​

 

[pc1966]

Muppetry at it's finest.

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?

 

[Rockingit]

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?

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.

 

[timhoward]

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?

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.

 

[Calebp43]

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?

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

 

[pc1966]

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?

Exactly that.

 

[swaRRR]

It's all the rage in the air con industry.
View attachment 99515

Can someone explain the joke to me.

 

[plugsandsparks]

Cannot its too funny, but i have been called out to a similar install where multiple a/c on a single RCD and i moved them on to individual RCBOs, because the single RCD would trip due to accumulation

 

[plugsandsparks]

And its a type AC .... lol

 

[timhoward]

Can someone explain the joke to me.

Look carefully at what each device is.