Log in to the online community
Advice is that the sum of cable lengths to an SPD must be less than 1m and should be less than 50cm ~ the shorter the better.
Standard MCBs are approximately 7cm across the terminals. Does anyone know what the typical inductance of these things is ~ because if internal conductors form a coil then certainly it will differ from the linear length of a straight, fine wire, fuse. Put it another way, what is the 'equivalent' length of a series connected MCB that should be discounted from 50cm ~ I doubt it is only 7cm?
However looking at Fig 534.8 in BS 7671 - it seems that the defined length to be measures deliberately excludes the OPD (and indeed any internal connections within the SPD package itself) - so my guess would be that whatever numbers they ground through to come up with 0.5m took into account typical impedances of an OPD and associated connections - so the likes of us need only worry about the actual cable lengths.
This is something I can measure properly when at work, but a quick flick here suggests that several hundred nano-henries is the ballpark - so a figure that eats into most of the half metre allowance at least.
A 13A fuse on the other hand is about 30nH - not much more than the wire equivalent of it's physical length.
It may not be in the regs or OSG yet, but using a fuse instead of MCB gives closer surge protection - or if you prefer you could trade the use of a fuse for longer cable run.
The results for SPDs are normally quoted without all the degradation of real wiring, so present an optimistic case- really the over-voltage will be higher and last longer due to the series inductance. Having had to do it at work, I can say that measuring surge waveforms is actually quite tricky, as you have to be very clear about where you are reading relative to just saying 'earth' will not do - for short pulses things that both earthed and are connected and at the same voltage for DC can be bouncing about quite independently of each other if the propagation time along wiring between them is not very short relative to the pulse period.
Still I thought some lab numbers might be of interest.
Thanks for this information. As I reported in my original post, I have an overhead TT supply, through a brand new 80 amp fuse in the DNO head, and analogue meter. Northern Powergrid is comfortable with the plastic Lewden enclosure containing a 100mA time delayed RCD and a Fusebox Type 1 Connection type 2 SPD, with line protection through a Mersen FR1 OAM4 OV32 (400 volt 32 amp) fuse. Total circuit length from the Line connection at the RCD through my components back to the Neutral connection at the RCD ~ plus the short cable from the Earth terminal on the SPD to the MET in my installation is 37cm. Difficult to see how it could be made shorter!
I notice that Fusebox now include a dedicated MCB for protection against the SPD failing permanently hard short circuit in some of their Consumer Units and it looks to me as if their total SPD circuit length is well over 50cm assuming the MCB adds just 7cm (its linear dimension).
At the risk of resurrecting a dead thread, I have just measured a GE series unit (B40 rating) and get ~150nHThanks for that Mike - very useful.
Does anyone know what sort of length of 6mm² or 16mm² single core that would compare with?
Wire diameter has almost a very modest effect on the shape of the magnetic field, which is where the energy is stored that makes the inductor do its thing. Bending the wire or bringing it near other wires is much more influential.
If however you can bring a cable carrying an equal and opposite current near (like L and N in twin and earth cable), then the two magnetic fields of the single wires overlap. In that case at least from a few wire spacings outwards, the fields of the two wires almost cancel, and the effective inductance is greatly reduced - but only for currents in the opposing direction - measure a twin wire push-push instead of push-pull and you get a very much higher self inductance. So the inductance from the input terminals through an RCD with the load side shorted is a lot less than the sum of the two paths L-input to L output and N input to N output measured seperately.
This is also why the AC voltage drop in high current trefoils is much better if each trefoil bundle has all 3 phases (and neutral) in it in preference to widely spaced bundles of all phase 1 cores, and another one bundle of phase 2 etc.
I suspect that the presence of the SPD will make no difference in terms of the supposed protection it is intendended to provide, but at least the regulatory requirement to actually fit one has been met, and the money is in the till of the provider.
The spd is a scam money making scheme along with part p. AFDD's and EV chargers
Mike, that immediately made me think of two way switching using 3 core and earth between the two switches
There is a related tendency of unbalanced wiring (by which I mean that flow and return currents are not kept close together) to act as an unwanted antenna, at higher frequencies this is significant, so layout can be really quite important, and can be something of a dark art to get right, either for interference getting in and confusing electronic controls, or noise from badly designed switch-mode supplies radiating out and jamming other things.
Even 'simple' things like phone lines are a lot quieter (and the internet more reliable) if lines are properly filtered and are routed as twisted pairs, and do not change to singles that part company for some distance and then rejoin late. Where there have to be sections that are opened up, these need to be kept away from other potential aggressors/victims
Apparently "Strappers" are known as "Travellers" in USA I`m told