Grid spikes tripping MCB?

[binky]

Now I just want to check that nothing would trip an MCB downstream of the said breaker, ie only a fault on cct protected by MCB would cause it to trip?

Got abit of an odd case, whereby an MCB is tripping. Apart from afew issues with the grid anyway which DNO have now fixed, it also seems to have stopped since a fella who likes fixing cars a few doors up has moved, so suspect it may be linked to welding equipment, big dodgy angle grinder.

The said MCB is actually for a solar system, independently linked to a mini CU wired into meter tails. The CU for the house is not suffering tripping. My opinion is that the constant restarting of inverter caused by grid spikes is what is actually upsetting the MCB, but I would be interested if anyone else has come across a situation where something on the grid side of an MCB has caused tripping?

 

[Marvo]

Now I just want to check that nothing would trip an MCB downstream of the said breaker....

You mean upstream?

I've never seen a power quality issue cause an MCB to nuisance trip, only RCD's. Apart from a direct lightning strike I'm not sure what other power anomoly could cause a high enough current to trip it. Much more likely to be a wire or busbar on the wrong side of the screw clamp or some kind of poor termination causing heat which nuisance trips the thermal mechanism.

 

[binky]

hello  @Marvo, thought you had got lost! :^O

upstream/ downstream, been a long day. AFAIC MCBs only trip for faults on the protected circuit, but many odd things can occur, so i thought I would double check I wasn't talking rubbish....

 

[Evans Electric]

As you say , odd things happen , never come across problems on the grid affecting MCBs  but ...we live & learn. 

The fact that the rogue angle grinder is removed from the equation is most pertinent I think.   

 

[binky]

I actually think it is the function of the solar inverter that was causing the MCB to trip, with the inverter having been 'upset' by a faulty grid cable and possibly the work of a car fixer. The cable was repaired some months ago, the flickering electrics and MCB tripping stopped after the car guy moved house. Co-incidence????

 

[Blue Duck]

Is it a Hager mcb? Or perhaps an mk? Or even worse a wylex?

:lol:

 

[Tony S]

This may be total rowlocks, breaking an inductive AC arc (welder, unsuppressed angle grinder, etc) will cause a spike on the supply side. If the inverter has MOV’s protecting the mains side it will trip the MCB.

I’ve had the spike from breaking a heavy inductive load causing flashover in a MCC panel taking 550A fuses out

 

[Marvo]

^^ The thought occured to me this would be possible in a commercial or industrial setup but the MOV's in most domestic appliances aren't likely to survive passing the kind of current that would be required to instantaneous trip an MCB.

hello  @Marvo, thought you had got lost! :^O

upstream/ downstream, been a long day. AFAIC MCBs only trip for faults on the protected circuit, but many odd things can occur, so i thought I would double check I wasn't talking rubbish....

Still around, just busy in real life at the moment.

Anything's possible and I wouldn't entirely rule out harmonics, phase angles, spikes, surges etc from an external source but I think it's very much a long shot that it's an event on the upstream side that's tripping an MCB. I'd focus on the MCB itself and the circuit(s) being supplied. (Yeah, I know that multiple ccts from the same ocpd are classed as being a single circuit)  

 

[Tony S]

MOV’s should survive a couple of kA, more than enough to kick a MCB in to action.

Depending on the load being drawn by other houses on the road over voltage could be absorbed making the fault appear random.

Too many if, buts and maybes.

 

[binky]

MOVs ?

 

[Tony S]

Metal Oxide Varistor.

As the voltage rises above a set point the resistance lowers

 

[Barx]

If it is part of a PV system, then surely the PV is the source and the Grid is the 'protected circuit'?

I have known many larger (4kW systems) inverters output >16A for a short time. and this can trip the breaker. If you look in most manuals for said inverters, it states to use 20 / 25A MCB's.

I would think it will be more a possibility if the array is oversized with the inverter used to cap the output. If for example the array is 5kW. Mr sunshine suddenly pops out from behind a cloud and the juice ramps up. te inverter gets all excited and start buzzing away. it takes a moment or 2 to think 'hang on, i'm not supposed to be doing this' and applied the brakes a bit. Much like a child going down a big hill on a push rod........ kinda

Well, this is my theory anyway

 

[binky]

bit of both really Barx..

I seen many systems on 16A MCB, which I personally think is a mistake, albeit the MCB should run at 16A and bit, for fairly sugnificant periods of time. You do get 'cloud edge effect' whereby the edge of the cloud can focus the suns rays and produce a small spike in irradiance at the panels. In this instance I suspect the spikes in grid votage are just upsetting inverter output, kind of banging on-off whilst the inverter is trying to output a good wattage and maybe it is outputting a little more than 16A from stored energy in the panels, rather than the nice smoothish ramp up you would normally get from a system.

 

[Tony S]

19 hours ago, Canoeboy said:



https://www.youtube.com/embed/CiBXN82vzNQ?feature=oembed

That’s a good explanation of how a MOV works.

They have been around a lot longer than said in the video particularly for MV end of line terminations.