Do SPDs need RCD protection ?

Ronnie-p · May 28, 2023

Hi there ,
Currently doing a college project where i need to select protective devices for the contract. I'm now at the point of selecting SPDs and had a couple questions. I have chosen MCG RCBOs for my circuit breakers in this project for all my boards apart from the board supplying distribution circuits which has MCCBs - so my SPDS will need to be same brand and the MCG SPDs require a 63A MCB to go with it. 1st of all its a commercial building -TN-C-S , do I need SPDs in every DB board ?.
Secondly do SPDs need RCD protection- as in does the MCB that goes with it need RCD protection. And if they do need RCD protection how would I go about installing them in RCBO boards.
If they need RCD protection and in my boards are RCBOs would I have to install a separate RCD to protect the MCB that's with the SPD or could I fit the SPD with an RCBO ?

cliffed · May 28, 2023

I would fit SPD’s @ the origin… but you can fit them elsewhere too
The Mcb supply to the SPD will provide protection… On a TT system mcb protection would be ok….most SPD ‘s are integral with the main switch

timhoward · May 28, 2023

Ronnie-p said:
Secondly do SPDs need RCD protection- as in does the MCB that goes with it need RCD protection. And if they do need RCD protection how would I go about installing them in RCBO boards.

Let's do this bit first. No. An SPD has to be able to dissipate surges to earth. If you had an RCD in front of it, after the first 20-30ma of the surge the RCD would trip and the surge would have to find another path rendering the SPD rather ineffective.
So most of the time it's as simple as install the MCB and install the SPD off it.
Here is a Fusebox example:
(there's also an earth wire from underneath the SPD to the CPC bar which you can't see)

[ElectriciansForums.net] Do SPDs need RCD protection ?

Ronnie-p said:
do I need SPDs in every DB board

Generally if they are all within the same building downstream of the same supply, no.
If a distribution circuit went to another building via an overhead supply then it's a different situation as lightning could strike the distribution circuit.

Ronnie-p · May 28, 2023

timhoward said:
Let's do this bit first. No. An SPD has to be able to dissipate surges to earth. If you had an RCD in front of it, after the first 20-30ma of the surge the RCD would trip and the surge would have to find another path rendering the SPD rather ineffective.
So most of the time it's as simple as install the MCB and install the SPD off it.
Here is a Fusebox example:
(there's also an earth wire from underneath the SPD to the CPC bar which you can't see)
View attachment 108448

Generally if they are all within the same building downstream of the same supply, no.
If a distribution circuit went to another building via an overhead supply then it's a different situation as lightning could strike the distribution circuit.

Ok thank you noted- and the building is as one but has 2 blocks- cafe block with a pool in basement and the hotel block with the rooms in. So I will put the SPD in The main board which then supplys the other boards and as it is a 3 phase board I'll put a 3 pole SPD on

HappyHippyDad · May 28, 2023

timhoward said:
Let's do this bit first. No. An SPD has to be able to dissipate surges to earth. If you had an RCD in front of it, after the first 20-30ma of the surge the RCD would trip and the surge would have to find another path rendering the SPD rather ineffective.
So most of the time it's as simple as install the MCB and install the SPD off it.
Here is a Fusebox example:
(there's also an earth wire from underneath the SPD to the CPC bar which you can't see)
View attachment 108448

Generally if they are all within the same building downstream of the same supply, no.
If a distribution circuit went to another building via an overhead supply then it's a different situation as lightning could strike the distribution circuit.

Off topic, but you've done well to get the conductors all nice and neat, especially with the annoying neutral leads from the RCBO's.

timhoward · May 28, 2023

HappyHippyDad said:
Off topic, but you've done well to get the conductors all nice and neat, especially with the annoying neutral leads from the RCBO's.

Cheers! I don't remember this job but it looks like I cut the N fly leads to length and put ferrules on. I must have been in a good mood.