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SPDs with MCB protection ~ Inductance ≡ 50cm
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37 Posts
Answered
Greetings,

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?

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17 Replies
AJJewsbury
1881 Posts
Good question - and short answer is that I have no idea what the impedance of an MCB would be - let alone that the high frequencies that approximate to a surge,

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.

  - Andy.
mapj1
2467 Posts
A very pertinent  question, and not something on the RADAR of the reg-writers I suspect - the answer will depend on the breaker rating -  a 5A MCB has a longer thinner winding than the  30A usually suggested for SPD use, and a 40 or 50A is lower inductance still.  
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.
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37 Posts
Thank you Mike; your response seems to validate what I merely suspected.
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wallywombat
288 Posts
Sounds like a reason to use SPDs that don't need an MCB (as long as backed by what the manufacturer recommends e.g. 100A fuse).
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37 Posts
Indeed Wally.  Would it be preferable to protect with (say) a 50 amp fuse to achieve discrimination with the DNO's cut out?

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wallywombat
288 Posts
The SPDs I was thinking of (such as the Wylex ones) have their own built-in one-shot fusible link - it's what turns the window from red to transparent, and gets replaced as part of the replaceable cartridge. I should imagine that link already discriminates well with the DNO  fuse.
mapj1
2467 Posts
At the risk of resurrecting a dead thread, I have just measured a GE series unit (B40 rating) and get ~150nH, measured at 25MHz i.e. with a signal  rise time comparable to the surges, this is quite lot lower than the inductance measured at audio frequencies (about 30% perhaps) so I presume the soft iron cores cannot keep up, and/or the interwinding capacitance conspires to shunt the fast stuff around the coil, so maybe it is not always as bad as feared.
Still I thought some lab numbers might be of interest.
wallywombat
288 Posts
The standard test waveform for a type 2 SPD has an 8us rise time, which I make to be roughly equivalent to 31KHz.
mapj1
2467 Posts
I take your point, but the reason we worry about line lengths is that real impuses include effects that are much faster - at 31 KHz half a meter of cable is neither her nor there. Now I've got it on the bench probably more to follow on Monday
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37 Posts
Greetings Mike and Wally,
       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).
 
whjohnson
320 Posts
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.
AJJewsbury
1881 Posts
At the risk of resurrecting a dead thread, I have just measured a GE series unit (B40 rating) and get ~150nH
Thanks for that Mike - very useful.

Does anyone know what sort of length of 6mm² or 16mm² single core that would compare with?

   - Andy.
mapj1
2467 Posts
Between 6 inches and a foot for that sort of inuctance, depending how much you bend it into a hairpin or loop (higher L) or straighten it out (lower L )
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.

regards
Mike
 
Jon Steward
20 Posts
whjohnson:
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.

Exactly
The spd is a scam money making scheme along with part p. AFDD's and EV chargers

ebee
830 Posts
"measure a twin wire push-push instead of push-pull and you get a very much higher self inductance "

Mike, that immediately made me think of two way switching using 3 core and earth between the two switches
mapj1
2467 Posts
The magnetic fields from some layouts of 2 way switching are much more noticable than others - however, at 50Hz you need a LOT of current before  it matters,  so unless you are either considering 2 way switching for hundreds of amps, or switching RF feeder cables, the balance or otherwise is not likely to be an issue for simple lamps.
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
ebee
830 Posts
Cheers, yes I know that T & E to SW! the 3 C & E from SW1 to SW2 might reduce induction loop hearing aid hum in comparison to the ancient tradition of L to SW1, SL to SW2 and T & E betwixt switches.

Apparently "Strappers" are known as "Travellers" in USA I`m told

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