Hi Folks,

So I raised this problem quite some time ago with regard to a caravan camp site where the owner was having the local pitch MCB being replaced at random due to over heating of the neutral connection on the MCB.

At the time I could not explain what was going on.

Since then I have been involved in a more serious example of the same but devices were being burnt out, cables were melting and protective device were tripping randomly.

The answer/ cause.............................................Harmonics.......................yep the one every body hears about but sometimes do not realise its a real world issue.

My conclusion is that modern mobile caravan/ campers etc. use numerous dc/ac inverters and converters for their vehicle and camping needs and often LED source illumination and other devices.

I suspect that these items vary in quality and may well be producing harmonics which are resulting in sub distribution harmonics and excessive neutral current well beyond the rated values of the devices specified for the "normal" expected running loads.

There are some good inline filters that could be fitted which prevent such harmonic distortion and effects and I for one will be giving this aspect of electrical design more consideration in future.

I have kept it short but believe thats the problem.

Cheers

DMB

I'm not sure it is harmonics - hence my suggestions for confirmatory checks above.

I'm also not sure what is meant by '2 phase' in this context - if it is really split phase (2 HV wires come in and the LV tx has a single primary, and a single480V secondary with the centre earthed) rather than a full fat 3 phase supply but with one phase unused (as I assumed initially), then the non cancelling argument does not apply, With bridge rectifier loads on split phase, the neutral currents do in fact cancel to a degree as the up and down phases actually do reach peak current of opposite polarity at the same time.

I think we are missing some of the details that would allow more intelligent comment.

Mike

With bridge rectifier loads on split phase, the neutral currents do in fact cancel to a degree as the up and down phases actually do reach peak current of opposite polarity at the same time.

Although some loads aren't as nicely behaved as traditional mathematical sine-wave assuming models might predict. Most caravans have what's basically a 12V SMPS come battery charger to run the 12V system and recharge the leisure battery. Rather than chopping up each cycle in the same way each time, it's perhaps more likely to run on a few-cycles-on then a few-cycles-off basis. Likewise triac controlled heating loads. Whether you still want to call that sort of thing "harmonics" is perhaps debatable, but it can have the same sort of effect on Ns. Given the on/off nature of the loads, the supply can often be "overloaded" in terms of instantaneous current without any overcurrent protection (or cable temperature) noticing, but the N might not get all the same "off" time to cool down.

But still it shouldn't be noticeable in wholly single phase parts of the system.

   - Andy.

But still it shouldn't be noticeable in wholly single phase parts of the system.

Quite - however hideously non sinusoidal the current, the L and N in a 1 phase loop, will see practically the same,(actually  on a properly wired campsite, to within 30mA RMS)

Only if the neutral incoming terminal has more than one cable in it, and there is a lot of current 'just passing by' not for that load, might there be a problem, and assuming L is similarly looped, then I'd not expect to see trouble just on N.

And the immediate fix for that is not to over fill the RCBO terminals, but to use either Henley blocks or sets of Line taps in a box depending on what suits the arrangement, not harmonic filtering.

Rather like 'Eddy Currents' and other well known Olympic skiers, an assumed problem with  'harmonics' may be invoked to cover cases that are too hard to explain - on its own, that does not make the explanation complete or correct - it needs verification.

BTW for the pedants copy book, repeated bursted multi-cycle synchronous loads could be described as sub-harmonic distortion - switching every second cycle would introduce  a term at  25 Hz, every tenth cycle at 5Hz  etc.  Now, thinking about waveform distortion in the spectral domain is not that useful sometimes, and the time domain view is clearer - for sub-harmonics this comment  is even more true.

There is a reason we talk about busses to Southampton arriving every 30 mins +/- some mins uncertainty rather than being a spectrum with a peak at 0.8 milli-Hz and some line width - both describe the same thing but the latter is not really a helpful description ...

Mike.

Is this effect similar to the comparison between a lift motor and a fan motor (ECA) both rated at 20 kW with a running current of 35 A and a starting current of 175 A. While the starting current of the fan motor can be disregarded, the duty cycle of the lift motor must be taken into account due to its frequent start-stop operations within a short period. This results in a significant heating effect from the duty cycle, thereby increasing the equivalent continuous current.

A 20 kW motor would not have a neutral!

There has still been no explanation why there might be any difference between L and N in a SP circuit.

Is this effect similar to the comparison between a lift motor and a fan motor (ECA) both rated at 20 kW

In terms of larger on-off currents being "thermally equivalent" to a smaller steady current, then yes, exactly so.  Say 200A being drawn 50:50 might be equivalent to only 100A (very roughly). The point then being that on/off on things on different phases won't necessarily be in sync, so the N might be getting 200A from one phase at one moment, then while that's off, be carrying 200A from another phase - so missing out on some of the cooling 'off' periods.

   - Andy.

Hi,

Could the burnt out neutral be due to lightning strike with fault on N-E SPD?

I notice this answer has the orange box around it, does that imply KDY have solved the mystery? 

Returning to the discussion on harmonic theory, I assume we are not referring to triplen harmonics ? Instead, we are addressing the odd multiples, although  less common in AC circuits. Andy/Mike. My understanding is that triplen harmonics only become problematic within three-phase distribution systems. particularly in three-phase four-wire circuits where the neutral conductor is involved. The issue with triplen harmonics lies in the combined currents circulating within the neutral conductor, which can lead to significant imbalances and potential overheating only in a three-phase, four-wire distribution system. So why is the neutral overloading on the line side of a single phase RCD?  

I know. My point was having to consider duty cycle impact on circuit calcs

I know. My point was having to consider duty cycle impact on circuit calcs