Crazy Troubleshooting Video

[tallgirl]

I stumbled across this and just had to share. I've never seen anything this messed up in my entire life.

 

[winnie]

Sort of like this puzzle:

Ungrounded Delta Ground Fault Through a Transformer

I have searched pretty hard for this scenario and haven't had any luck, hopefully you guys can help. I work at an industrial plant with 480V three phase ungrounded deltas for most MCCs. The MCCs all have ground fault indicating lights and the other day we had B Phase light go out in MCC #1 and...

forums.mikeholt.com

The ground fault was actually on the 480V side, but there was an interlock that was disconnecting the ground fault when a 240V breaker was turned off. Since flipping the 240V breaker removed the ground fault, lots of time was spent trying to figure out how a ground fault could pass through a 480:240V isolation transformer.

No comments on the junction boxes floating around under the floor and behind appliances.

-Jon

 

[AC\DC]

We must be excessively paranoid around here, when ever I see UK video on electrical it seems like DYI stuff around here and I don't know if they have any more electrical deaths then us. I do like there panels look cute.
Not criticizing them probably compliant for them.

 

[GoldDigger]

Ring circuits!

 

[ptonsparky]

Ring circuits!

Dang nabbit!
My hearing aids don't help much with accent and it was longer than two minutes. I was going to watch it in the early AM when coffee is still fresh in my body.
Guess I'll still watch.

 

[jaggedben]

Yeah, still watch. The actual problem has nothing to do with ring circuits. Still not sure I totally understand it, but I know that much.

 

[ptonsparky]

Interesting meter.
Yes, it does look like DIY work. Plumbing included.

 

[Rock86]

I have no experience with UK wiring, but that was a pretty wild fault. That was a pretty sweet meter he had though.

 

[drcampbell]

I thought the UK used single-ended, single-voltage services that don't have a "neutral" wire?

 

[ptonsparky]

I thought the UK used single-ended, single-voltage services that don't have a "neutral" wire?

Single voltage with neutral by the looks of it. No MWBC.

 

[jaggedben]

I'm guessing they bring one leg of a wye system so it's neutral in that sense even if the full system isn't present at each service (or whatever they call it, 'consumer unit'?).

 

[wwhitney]

For those interested in the discussion who don't like videos, the upshot, per the narrator, is this:

A previous electric boiler (to heat water for domestic use and for space heating) was supplied with a 2-wire circuit that only had switched power from a thermostat/controller. A replacement boiler (probably with some fancier electronics) needed both constant power and switched power. So the installer of that boiler grabbed constant power from a nearby receptacle circuit that was on a different breaker. They used L-N from the receptacle circuit and only switched power from the original circuit.

The result was that when just the receptacle circuit was shut off, but the controller called for heat, the boiler circuit backfed the receptacle circuit line conductor and shocked the electrician. The fix was to refeed the boiler with a 3-wire circuit from the controller.

Two points that at first glance are inconsistent with the above description, with possible explanations:

1) The electrician stated all the breakers were RCD (?), i.e. they had (30 ma?) ground fault detection. So if he got shocked, it should have tripped the boiler breaker, since the return current was not on the boiler breaker. This was not mentioned (IIRC). But perhaps the return current was below the trip threshold?

2) In the system state where the electrician was shocked, his meter on the receptacle circuit read voltage L-G and N-G, but not L-N. I would expect it to have read normally. But perhaps since the breakers are RCD, they actually disconnect the neutral conductor when off. In which case the raised N-G voltage would have been due to capacitive coupling with the energized L conductor?

[Note that the boiler circuit also supplied upstairs receptacles, but I've just called it the boiler circuit to make the distinction between circuits.]

Cheers, Wayne

 

[jaggedben]

For those interested in the discussion who don't like videos, the upshot, per the narrator, is this:

...

2) In the system state where the electrician was shocked, his meter on the receptacle circuit read voltage L-G and N-G, but not L-N. I would expect it to have read normally. But perhaps since the breakers are RCD, they actually disconnect the neutral conductor when off. In which case the raised N-G voltage would have been due to capacitive coupling with the energized L conductor?

...

Yeah, this part confused me somewhat. I think you must be correct that the breaker for the downstairs receptacle circuit disconnected the neutral, because otherwise I didn't see how the neutral on the boiler plug became energized to ground without being a straight short circuit with both circuits turned on. My best theory is that with the boiler circuit energized, the appliance passed the voltage (almost?) straight through to the neutral on the plug, thus energizing the disconnected neutral wire. All in all, somewhat similar to how here in the states you'd have voltage from a disconnected neutral wire to ground if you lift one line and neutral of an MWBC while the other line is still energized and load turned on.

Also not sure I can explain why the voltage was 170, not 230, and why it went up when other items were unplugged. Not that I've tried to puzzle it out completely. (Neither did our hero in the video, apparently.)

 

[GoldDigger]

Yeah, this part confused me somewhat. I think you must be correct that the breaker for the downstairs receptacle circuit disconnected the neutral, because otherwise I didn't see how the neutral on the boiler plug became energized to ground without being a straight short circuit with both circuits turned on. My best theory is that with the boiler circuit energized, the appliance passed the voltage (almost?) straight through to the neutral on the plug, thus energizing the disconnected neutral wire. All in all, somewhat similar to how here in the states you'd have voltage from a disconnected neutral wire to ground if you lift one line and neutral of an MWBC while the other line is still energized and load turned on.

Also not sure I can explain why the voltage was 170, not 230, and why it went up when other items were unplugged. Not that I've tried to puzzle it out completely. (Neither did our hero in the video, apparently.)

I think he may have been seeing 170 as the result of static (always on/plugged in or capacitive) loads on the two dircuits. One circuit energized the neutral but loads on the "open" circuit bled off some of the voltage. But capacitive loads could not have supported much current.

 

[hbiss]

That test set looks more like it belongs to the cable guy. But anyway, I never watch UK or foreign electricians because the installations look more DIY than anything we have here. And ring circuits? Nothing would pass the NEC or UL as far as I can see.

A replacement boiler (probably with some fancier electronics) needed both constant power and switched power. So the installer of that boiler grabbed constant power from a nearby receptacle circuit that was on a different breaker. They used L-N from the receptacle circuit and only switched power from the original circuit.

What the heck kind of boiler is that? I rest my case.

-Hal

 

[synchro]

A previous electric boiler (to heat water for domestic use and for space heating) was supplied with a 2-wire circuit that only had switched power from a thermostat/controller. A replacement boiler (probably with some fancier electronics) needed both constant power and switched power. So the installer of that boiler grabbed constant power from a nearby receptacle circuit that was on a different breaker. They used L-N from the receptacle circuit and only switched power from the original circuit.

The result was that when just the receptacle circuit was shut off, but the controller called for heat, the boiler circuit backfed the receptacle circuit line conductor and shocked the electrician.

With electronic controls the current drawn on the switched power coming from the thermostat controller might be quite low, and perhaps significantly less that the 30mA needed to trip an RCD. This could explain why the current from the hot of the switched power circuit which returned through the neutral of the constant power circuit was apparently not tripping the RCD's of either circuit (the electrician was only called to move a socket). And it could also explain why the RCD apparently didn't trip either when the electrician got shocked.

If there are some intervening electronic circuits between the switched power and constant power inputs to the boiler, there could be a significant voltage drop before one input could backfeed into the other. Hence the L-E and N-E could plausibly be the measured 170V with some minimal capacitive loading to pull it down as mentioned by GoldDigger.

 

[wwhitney]

With electronic controls the current drawn on the switched power coming from the thermostat controller might be quite low, and perhaps significantly less that the 30mA needed to trip an RCD.

Ah, right, I failed to realize that since the RCD wasn't tripping during normal usage conditions, that implies that all the power to the heating element was coming from the constant power circuit, as that is the only circuit whose neutral was in use.

That also means the electrician could have fixed the different circuits problem by just adding a relay (since the original switched power cable had its own neutral). But perhaps the final result was superior, to have the boiler hardwired from the original boiler circuit, rather than cord and plug connected to the nearby receptacle circuit.

Cheers, Wayne

 

[hbiss]

What the heck kind of boiler is that? I rest my case.

My point is a boiler needing two sources of power is just asking for trouble. The control voltage should have come from the boiler itself, not a separate circuit.

A previous electric boiler (to heat water for domestic use and for space heating) was supplied with a 2-wire circuit that only had switched power from a thermostat/controller. A replacement boiler (probably with some fancier electronics) needed both constant power and switched power. So the installer of that boiler grabbed constant power from a nearby receptacle circuit that was on a different breaker. They used L-N from the receptacle circuit and only switched power from the original circuit.

-Hal

 

[jaggedben]

My point is a boiler needing two sources of power is just asking for trouble. The control voltage should have come from the boiler itself, not a separate circuit.


-Hal

Seem's that's how it's designed. The guys who bolloxed it up miswired it, right?
It does seem weird to use line voltage for a control circuit though.

 

[synchro]

Seem's that's how it's designed. The guys who bolloxed it up miswired it, right?
It does seem weird to use line voltage for a control circuit though.

Yes, and a 230V to ground/earth line voltage at that.