Block of 200+ flats, eleven single channel hot water timers blown this week alone.

littlespark · Aug 20, 2021

Might be hard to tell now, but what were the terminations like?
im thinking these were all fitted when the block was new, just wondering if maybe the same- not very effective- electrician did the connecting and left a loose screw in these particular flats.

clutching at straws, but always something to think about if all other avenues of investigation are exhausted.

LukeD · Aug 20, 2021

littlespark said:
Might be hard to tell now, but what were the terminations like?
im thinking these were all fitted when the block was new, just wondering if maybe the same- not very effective- electrician did the connecting and left a loose screw in these particular flats.

clutching at straws, but always something to think about if all other avenues of investigation are exhausted.

Has to be a factor.... The ability for someone to just make a mistake/miss something is super high

GBDamo · Aug 20, 2021

Yozza19 said:
I would also see if the affected units are on the same phase still can’t rule out floating neutral

If love to, I fact already discussed the possibility with site but that is a herculean task without drawings, access and someone on site who has a clue.

GBDamo · Aug 20, 2021

littlespark said:
Might be hard to tell now, but what were the terminations like?
im thinking these were all fitted when the block was new, just wondering if maybe the same- not very effective- electrician did the connecting and left a loose screw in these particular flats.

clutching at straws, but always something to think about if all other avenues of investigation are exhausted.

Possible but it's the time frame that leads me away from workmanship, coupled with the recent other electrical issues.

I don't know for sure but the block looks to be ten year old and for bad workmanship to start to show up so late in the day then all within a week makes me think other factors are at play.

Mikegh · Aug 20, 2021

GBDamo said:
If love to, I fact already discussed the possibility with site but that is a herculean task without drawings, access and someone on site who has a clue.

Wander lead

Joking

GBDamo · Aug 20, 2021

Here's some photos @Lucien Nunes and others.

The terminals and cables are clean as a whistle.

This is the culprit

Capacitor(?) has blown.

plugsandsparks · Aug 20, 2021

GBDamo said:
They are rated at 16A. The hot water cylinders are on a dedicated circuit off their own 16A MCB, isolated via a 20A double pole switch, length is a couple of meters.

Couldn't see a rating on the element but looking into that now.

Edit to add,

The elements are 3kW but looking at the plumbing I think they're on a central hot water system as there seems to be a coil on the cylinders.

I'm back there tonight two replace a couple and grab some more info.

Would definitely clamp it , you may get over 14 Amps if lucky...lol but is stuff that good nowadays. BTW i only use Timeguard products for this type of stuff as fed up changing crap out after a few years

freddo · Aug 20, 2021

Looks like a metal oxide varistor connected across the supply, If several blew at once this is a sure sign of overvoltage on the supply. A MOV is normally high resistance, but it's resistance drops as the voltage increases to stop voltage spikes. A sustained overvoltage such as from a supply fault will quickly overheat and destroy an MOV.

Yozza19 · Aug 20, 2021

freddo said:
Looks like a metal oxide varistor connected across the supply, If several blew at once this is a sure sign of overvoltage on the supply. A MOV is normally high resistance, but it's resistance drops as the voltage increases to stop voltage spikes. A sustained overvoltage such as from a supply fault will quickly overheat and destroy an MOV.

Would definitely put my money on a floating neutral as some stage some Dno have a search function on there site to advise if there have been any incidents in the locality

Lucien Nunes · Aug 20, 2021

Yup, it sure does look like an MOV that wasn't big enough absorb the available energy in the overvoltage event. When that happens, it can initiate an arc that lasts until the fuse blows, which can dissipate so much more energy that it makes it hard to identify whether the trigger event was an impulse or a prolonged excursion above clamping voltage.

Assuming the building is served by conventional urban underground distribution the latter seems the most likely and I would say the likelihood of a cable fault causing both the timer damage and the flickering / outage events was very high. Other items in the properies with MOVs (and even SPDs) might also have been compromised, but if the MOVs in the timers had a slightly lower clamping voltage then they might have all gone phut before many of the remainder showed signs of distress.

The sub might be specific to the estate or building, in which case any information you can get from the DNO about recent faults might nail it.

GBDamo · Aug 20, 2021

Thanks guys.

Getting stuck into some tinnys now so will pick this back up tomorrow.

Cheers

GBDamo · Aug 21, 2021

My thinking of what we have so far.

The catastrophic failure or the MOV (thanks) in the controller would point to a large over voltage situation.

It is most probably not environmental but a lost Neutral.

Without further information this could be anywhere internal or external.

So I was thinking to approach it on two fronts, to contact the DNO and report a possible Neutral fault whilst surveying the site, reported faults and drawings to see if a pattern can pinpoint the location.

So in a normal fault free situation the installation should look like.

If the fault is in the below position then I should only be hearing of power outages in flats 7, 8 & 9 but not over voltages or the MOV failures I'm seeing.

If the fault is in the below position then flats 4, 5 & 6 and flats 7, 8 & 9 will see voltage fluctuations between 0V and 400V depending on their respective loads, this situation could cause the MOV failures.

And finally, in the below image the fault would be evident across all phases and all flats with under and over voltages. Whilst there are sections of the internal installation that could contain the fault; in this scenario the fault could also be external.

Ii actuality I have already informed the client of the liklihood of a serious fault existing and that it may be internal or external and they will be picking it up from here.

I'm quite disappointed really as its quite interesting to see how far you can fault find on paper if you have enough information.

Lucien Nunes · Aug 21, 2021

Your explanation of the possible scenarios is correct but the configuration in pic.3 is unlikely to exist in practice. Each single phase service will be derived from a 3-phase busbar, with neutrals normally serving one or three phases but seldom two.

If the fault was a cable strike, the neutral may have not merely been broken but also shorted or arced over to a line. A variety of different voltages may have been impressed on the loads in quick succession as the fault developed.

LukeD · Aug 21, 2021

I am loving this thread...some very clever replies

marconi · Aug 21, 2021

I don’t think the burned out disc is an MOV. The timer uses a transformer less way of producing low voltage dc to operate the electronics. I reckon one can see a bridge rectifier - the white block component, and a disc capacitor in series with a resistor of suitable power rating together forming a mains to low voltage dropper. The placement of the resistor and the disc capacitor makes me think their right hand ends are connected to Lin and Nin respectively and their left hand ends to the ac inputs of the bridge rectifier which are normally the inner two leads for this style of rectifier. There is a smoothing capacitor to the left of the bridge rectifier. I reckon the gold rimmed cylindrical component above the relay is a battery for keep alive in the event of power cut and accumulator to handle short duration current surges when the relay turns on.

Strima · Aug 21, 2021

marconi said:
I don’t think the burned out disc is an MOV. The timer uses a transformer less way of producing low voltage dc to operate the electronics. I reckon one can see a bridge rectifier - the white block component, and a disc capacitor in series with a resistor of suitable power rating together forming a mains to low voltage dropper. The placement of the resistor and the disc capacitor makes me think their right hand ends are connected to Lin and Nin respectively and their left hand ends to the ac inputs of the bridge rectifier which are normally the inner two leads for this style of rectifier. There is a smoothing capacitor to the left of the bridge rectifier. I reckon the gold rimmed cylindrical component above the relay is a battery for keep alive in the event of power cut and accumulator to handle short duration current surges when the relay turns on.

A picture of the underside of the PCB would be handy.