SPD on RCD acceptable or not?

iDragonBox · May 18, 2023

Inspecting and testing a consumer unit as part of my training and have noticed that the circuit breaker for the SPD is on the RCD? I am unsure weather this is acceptable as I haven't come across this configuration and I am pretty new understanding SPDs could one of you more experienced people explain to me why it is or isn't it would really help with my understanding and training.

[ElectriciansForums.net] SPD on RCD acceptable or not?

pc1966 · May 18, 2023

No, it should not be off a 30mA RCD as it might trip it on a surge.

They are fine after a 'selective' RCD (once common as a TT incomer) as that will ignore short spikes of voltage leading to imbalance current (the sort of thing SPD deal with).

iDragonBox · May 18, 2023

pc1966 said:
No, it should not be off a 30mA RCD as it might trip it on a surge.

They are fine after a 'selective' RCD (once common as a TT incomer) as that will ignore short spikes of voltage leading to imbalance current (the sort of thing SPD deal with).

I just asked one of the people I am training with and they said it was fine as this brand of SPD no earth leakage so can be installed downstream of an RCD/ RCBO. Would this then be fine if he is correct?

littlespark · May 18, 2023

I hope you also noticed the missing busbar shield and unbalanced load between the two RCDs.

This is an image from screwfix site of that type of board, (BG) where the SPD is through an MCB, not an RCD. I can only assume your board has been retrofitted with an SPD, not original

mainline · May 18, 2023

iDragonBox said:
Inspecting and testing a consumer unit as part of my training and have noticed that the circuit breaker for the SPD is on the RCD? I am unsure weather this is acceptable as I haven't come across this configuration and I am pretty new understanding SPDs could one of you more experienced people explain to me why it is or isn't it would really help with my understanding and training.

View attachment 108233

Surge protective devices (SPDs) should wherever possible be installed upstream of RCD to prevent unwanted tripping caused by transient overvoltages

Also 534.4.5.1

timhoward · May 18, 2023

iDragonBox said:
I just asked one of the people I am training with and they said it was fine as this brand of SPD no earth leakage so can be installed downstream of an RCD/ RCBO. Would this then be fine if he is correct?

The way an SPD works is by diverting the surge to earth. This will cause a trip leaving the surge free to find another route to earth.
Higher quality brands actually spell out that only a selective RCD may be fitted upstream, e.g. Schneider:

The BG spec sheet doesn't talk about coordination of devices, but does confirm my comment:

AF43DD5FF · May 18, 2023

pc1966 said:
No, it should not be off a 30mA RCD as it might trip it on a surge.

They are fine after a 'selective' RCD (once common as a TT incomer) as that will ignore short spikes of voltage leading to imbalance current (the sort of thing SPD deal with).

This got me thinking; on a TT arrangement with an all RCBO board and no upfront RCD you'd need to provide a suitable RCD for the SPD?

What about boards whose SPDs don't require an MCB for protection presumably they would likely require a suitable RCD for fault protection if installed on a TT supply?

nicebutdim · May 18, 2023

AF43DD5FF said:
This got me thinking; on a TT arrangement with an all RCBO board and no upfront RCD you'd need to provide a suitable RCD for the SPD?

What about boards whose SPDs don't require an MCB for protection presumably they would likely require a suitable RCD for fault protection if installed on a TT supply?

I'm not following your reasoning. Why would you need to provide RCD protection for an SPD?

timhoward · May 18, 2023

nicebutdim said:
I'm not following your reasoning. Why would you need to provide RCD protection for an SPD?

I think he is considering a L-E fault within the SPD itself not clearing the main supplier fuse.
My understanding is that the construction method makes this virtually impossible.

pc1966 · May 18, 2023

The SPD type for TT have one device L-N and another N-E to reduce the risk of a L-E short needing some means to clear it (e.g. an RCD up-front). Often they are used in most cases due to the 'safe' arrangement.

The SPD designed for TN systems have all devices from live-E but they don't have the same risk of L-E causing a live installation.

Personally, and due to being a bit paranoid (partly my nature, partly the Voice of Experience), I would use a 100mA selective RCD up-front of an all RCBO board just in case, but if properly installed and correct SPD used then it is not absolutely necessary.

pc1966 · May 18, 2023

In the regs they are refereed to as CT1 and CT2, but for a SP system they are often denoted as 2+0 and 1+0 (just to add confusion with Type 1 and Type 2 of course, as we have Type A and Type B RCDs along with B-curve MCB, etc.) or for 3P as 4+0 and 3+1, etc.

See Table 534.5 under regulation 534.4.6

timhoward · May 18, 2023

pc1966 said:
In the regs they are refereed to as CT1 and CT2, but for a SP system they are often denoted as 2+0 and 1+0 (just to add confusion with Type 1 and Type 2 of course, as we have Type A and Type B RCDs along with B-curve MCB, etc.) or for 3P as 4+0 and 3+1, etc.

See Table 534.5 under regulation 534.4.6

You bring up an interesting point.
A Fusebox SPD kit has the L-N and N-E configuration and is advertised as suitable for TN and TT earthing systems. So I get quite used to not having to think too much about this topic. It's left me wondering whether any brands in the mid-range domestic market make different devices for TN and TT systems, mainly so I can store away the fact that I need to think if I deviate from Fusebox!

nicebutdim · May 18, 2023

timhoward said:
I think he is considering a L-E fault within the SPD itself not clearing the main supplier fuse.
My understanding is that the construction method makes this virtually impossible.

I'd be more concerned with ensuring the SPD was suitable for use with TT earthing, than considering an upstream RCD, although it seems that many now are.

Edit: above point covered in significant detail in subsequent posts.

AF43DD5FF · May 19, 2023

nicebutdim said:
I'm not following your reasoning. Why would you need to provide RCD protection for an SPD?

timhoward said:
I think he is considering a L-E fault within the SPD itself not clearing the main supplier fuse.
My understanding is that the construction method makes this virtually impossible.

Yes, this was what I was trying think through in my head.

pc1966 · May 19, 2023

timhoward said:
You bring up an interesting point.
A Fusebox SPD kit has the L-N and N-E configuration and is advertised as suitable for TN and TT earthing systems. So I get quite used to not having to think too much about this topic. It's left me wondering whether any brands in the mid-range domestic market make different devices for TN and TT systems, mainly so I can store away the fact that I need to think if I deviate from Fusebox!

I think the argument for the 2+0 configuration for the TN-S case is under lighting surge conditions both L & N will both be disturbed by a similar amount w.r.t. E, so clamping both that way keeps the L-N difference down far more than the 1+1 configuration used for TT.

However, for the 1+1 configuration used for TT you get better clamping of L-N voltage for other sorts of disturbances such as fault-clearing or big motors being switched where the underlying surge voltage difference is between L & N anyway.

For TN-C-S is makes little difference given that N & E are tied together typically only a meter or so from the SPD