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Big argument with a newly qualified electrician with all the necessary qualifications
Had to give him a hand later on during the day, nice DB fitted &to be fair quite tidy too , with SPD…It’s a TT System with a good Ze 24.8 ohms.
All Rcbo’s except for the Mcb supplying the SPD, which I pointed out to change that for a RCBO..
His argument was it didn’t present a dangerous situation being local to the DB..
My argument it’s a TT system not compliant & a C2
 
Tricky one. The issue with a RCBO feeding it is both the risk it trips on a surge and nobody notices, and also that the sense coil of the RCBO will marginally degrade the SPD's effectiveness.

But the flip-side is with only a MCB you are depending/trusting that the SPD is very VERY unlikely to have a L-E short. That ought to be the case electrically given they are usually MOV L-N and then GDT N-E but it also depends on the internal mechanical layout being such that under serious thermal stress from a nearby lightning strike it won't melt and short out.

If it were me and nobody worrying about the pennies I would have gone for a 100mA delay RCD incomer and then the lowest inductance MCB possible (e.g. 32A D-curve if permitted) or better still one of the SPD that is OK for 100A plus supply fuse so no MCB needed at all. But back in the real world...
 
SPD's should not be downstream of an RCD, the RCD will trip when the SPD starts to dump the surge to earth and the surge protection won't work.

Any SPD fitted to a TT system should be of a type that the manufacturer has approved for use on a TT system.

Whilst his reasoning is incorrect, he is right that an RCD should not be fitted.
 
But the flip-side is with only a MCB you are depending/trusting that the SPD is very VERY unlikely to have a L-E short.

Shouldn't be a problem using an SPD which is designed for a TT system, these will as you have said have protection L-N and N-E so that a failure causes a L-N short and operates the MCB.
An SPD designed only for TN systems will have L-E and N-E protections.
 
SPD's should not be downstream of an RCD, the RCD will trip when the SPD starts to dump the surge to earth and the surge protection won't work.
Strictly speaking it will be the following surge that is not protected. The RCD/RCBO won't have opened in the microseconds/millisecond of the surge that trips it!
 
when the SPD starts to dump the surge to earth
You've got me thinking, and I don't mind looking like an idiot here...
How well can an SPD dump a surge to earth through 24.8 ohms Ze?

In other words, for an SPD to do anything useful at all does there need to be a low impedance earth path?
 
You've got me thinking, and I don't mind looking like an idiot here...
How well can an SPD dump a surge to earth through 24.8 ohms Ze?

In other words, for an SPD to do anything useful at all does there need to be a low impedance earth path?
It reduces the L-N-CPC differences for appliances within the installation so they have much less chance of damage.

I think LPS are usually below 5-10 ohm or so, but also you have to remember as well as keeping the CPC close to "true Earth" 0V they also pull up the local soil to closer, so again it is about keeping the local difference down.
 
What’s the correct SPD for the TT system
They are normally the sort with one device L-N (typically Metal Oxide Varistor) and another device from N to E (typically Gas Discharge Tube). Sometimes this is referred to as '1+1' configuration for single phase, or '3+1' for 3-phase + N supplies, which is confusing as it has nothing to do with 'type 1' (for high energy direct surges) or 'type 2' (for lower energy induced surges). I think the vast majority of devices sold are of that type as it is also suitable for TN systems.

Some suppliers list them by suitable earthing type:

For TN-S and TN-C-S systems you can get alternative configurations where all protective devices are live to E (e.g. L-E and N-E connections). Sometimes this is referred to as '2+0' configuration for single phase, or '4+0' for 3-phase+N. Here is a comparison of two others showing the differing choices:


In most cases the TT style are fine for adequate protection and no need to worry about applicable choice. The TN style can offer lower clamping for voltages to Earth when applicable, but for most electronics what kills them is the L-N difference and you could argue the TT configuration is better for that.
 
Thanks for all comments….seems like an interesting outcome… especially in the factory assembled DB/CU where no Mcb required for the SPD.
Looks like the outcome …here it’s ok to leave the Mcb in situ
 
Just to add that on looking for something else, I found the connection types (1+1 vs 2+0 sort of idea) in the 18th AM2 under 534.4.3 and the related table to determine suitability with TT under Table 534.5 where the 2+0 is only suitable down-stream of a RCD, while the 1+1 style is acceptable.
 

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