CT1 and CT2

In reality current just flows, and to divide it into common or differential mode is a convenient mathematical  simplification, however when the SPD breaks over during a transient, it is indeed all about where the current goes first.


'all mode' in this context means that current can get round during an over-voltage between any pair of wires LLLNE .


All tests are really examples of common mode, after a fashion, but the choice of wires for the pair to be stimulated varies, and some documents apply a mythical status too the PE/CPC.


Never shown on the simple drawings is the fact that earth is not a potential, nor an infinite source and sink of current with no voltage variation,  in reality it is just another  wired connection, and one with considerable time delay and inductance as far as pulses of a few microsecond duration are concerned.


The test waveform is 8uS rise, 20uS fall, and the SPD itself must pass either 10 or 20kA peak, and at no time drop more than 1.2kV across its terminals, however real pulses come in all shapes and sizes, some much shorter, some much longer, and in higher and lower currents. The ones that are longer and in higher currents usually lead to SPD failure...

I have various pages open on my Ipad that I am still trying to get to grips with.


This one seems to make the most sense, the wiring arrangement needs to take account of if there’s a RCD upstream or downstream of the SPD.

https://www.lsp-international.com/surge-protection-devices-spds-and-rcd/

Quote:

 

An SPD configuration based on connection type 2 (CT2) is required on a TT earth arrangement if the SPD is upstream of the RCD. The RCD being downstream of the SPD would not operate should the SPD become defective.

Well in the CT2 arrangement, if the L-N device fails, the fuse will disconnect, If the N-E device fails, no dangerous voltage should be exposed, or at least a lot less than if there was an L-E on that failed.


Generally like all semiconductors after failure , the solid state transorb devices become a low resistance  - Gas discharge devices normally do not, as when they finally fail the electrodes inside melt and fall off, or the case cracks and the gas escapes. Gas devices need the volltage to fall to near zero for the arc to go out, so can only be used L- anything with a fuse in series.


regards Mike.

I have took part in a couple of Webinars on the subject of SPD and tried asking questions to get a clear image of the connections in my head, didn’t quite get there. Reading that article last night I think I cut through  the overload of information and got back to basics. 


If the installation is TN-C-S with a hardwired link between neutral and earth at the intake and that is where the SPD is you don’t need a a device between the neutral and earth, because they are hardwired together.


If the installation is TT there is not a hardwired link between neutral and earth at the intake and you cannot install one, because you would be making a conversion to TN-C-S which is not permissible or good engineering, so you need a device between the neutral and earth to keep the separation between them, whilst allowing a discharge of electricity if required.


A TNS installation also requires a device between neutral and earth to keep the separation, so it is not converted to TN-C-S.


Does that sound like a reasonable summary?


Then there’s also the issue of blowing fuses or tripping trips that are protecting the SPD devices, the suppliers main fuse and/or additional over current protective devices installed in the enclosure with the SPD.


Andy B.

And with the TT installation you don’t want a SPD that will fail leaving a connection between neutral and earth leaving it as TN-C-S.

And with the TT installation you don’t want a SPD that will fail leaving a connection between neutral and earth leaving it as TN-C-S.

That risk is there if the N-PE the SPD fails to short - as there's no guarantee that any OPD will operate to isolate it with the small voltage difference between N and PE.


With CT2 we connect L-N and N-PE so at least a N-PE short won't be as immediately dangerous as a L-PE one.


I have a feeling that many manufacturers pick a SPD technology for the N-PE SPD that is less inclined to fail short-circuit (gas dischage types?) but I've not found that written anywhere yet.


The other option is to put the SPD after the first tier RCD - but then the surge current has to go through the RCD - not only not protecting the RCD from damage but risking more nuisance tripping too. Rock and hard place.


   - Andy.

What is the answer to the original question?

I must confess, I`m struggling to get my head around all the subtle differences