explain this!

slavetothegrind

110v site transformer, small 2 socket type, plugged into a socket in industrial unit.
Transformer fails internally and power goes off to socket.
Fuse in plug ok, rcbo in mdb up, power not present into rcbo.
Phase down, 63a neozed in board switchfuse ok.
Go to meterbox on site and check switchfuse for unit, neozed 63a blown on phase in question.

Rcbo no longer functional, it's fuct, wont pass through and trip test fail so for the bin.

Can anyone enlighten me as to how the fuse at the meter was the one to go?

I can only guess weak fuse to start with? this is a new installation, ecssa electrician tested and certified.

cheers folks!

Bruthal

slavetothegrind wrote: »

110v site transformer, small 2 socket type, plugged into a socket in industrial unit.
Transformer fails internally and power goes off to socket.
Fuse in plug ok, rcbo in mdb up, power not present into rcbo.
Phase down, 63a neozed in board switchfuse ok.
Go to meterbox on site and check switchfuse for unit, neozed 63a blown on phase in question.

Rcbo no longer functional, it's fuct, wont pass through and trip test fail so for the bin.

Can anyone enlighten me as to how the fuse at the meter was the one to go?

I can only guess weak fuse to start with? this is a new installation, ecssa electrician tested and certified.

cheers folks!

Almost sounds like some sort of power surge or voltage spike which blew the main fuse and the transformer with it, rather than the transformer blowing the main fuse,

Or the main fuse was already right at its limit from other circuits.
Strange things sometimes happen electrically which are hard to find a reason for.
A fella i was workin beside before dropped a piece of wire into an old panel and shorted 2 phases, but it managed to blow all 3 fuses, a few strange things happen with electricity. Maybe someone else has a better explanation.
What was the failure in the transformer?

slavetothegrind

Transformer meters dead short.

There was no other load on that phase at the time guaranteed!

wierd alright!

Bruthal

slavetothegrind wrote: »

Transformer meters dead short.

There was no other load on that phase at the time guaranteed!

wierd alright!

Another that can cause that with a socket circuit is a reverse polarity on the socket circuit itself, then a short in transformer from neutral to earth will actually go down the neutral pin into socket and bypass the plug fuse etc,

Rcbo would of tripped then though unless the large short circuit stuck it.

M cebee

check Zl and PFC at the 13amp socket and the rcbo breaking capacity

as robbie said reverse polarity on the socket may bypass the fuse on a L-E short circuit

some odds against all that mind you!

any other equipment damage or neutral /overvoltage issues

2011

It sounds like you had a phase to earth or a phase to neutral shourt circuit. When this happened the fault current would have been very large. This large fault current could have fried your RCD.

When very large fault currents flow fuse discrimination may not always work the way you intend!

Nothing you have described would suggest to me that there is anything wrong with the wiring.

M cebee

2011 wrote: »

It sounds like you had a phase to earth or a phase to neutral shourt circuit. When this happened the fault current would have been very large. This large fault current could have fried your RCD.

When very large fault currents flow fuse discrimination may not always work the way you intend!

Nothing you have described would suggest to me that there is anything wrong with the wiring.

hmm i think it warrants investigation

if a short circuit has bypassed the plug fuse and rcbo

he'll have to report back the PFC at the socket anyway to get a better idea

Bruthal

2011 wrote: »

It sounds like you had a phase to earth or a phase to neutral shourt circuit. When this happened the fault current would have been very large. This large fault current could have fried your RCD.

When very large fault currents flow fuse discrimination may not always work the way you intend!

Nothing you have described would suggest to me that there is anything wrong with the wiring.

It would be doing exceptionally well to blow a distant 63amp neozed and leave the 13amp plug fuse intact though if it was the transformer that blew the main fuse.

Bruthal

M cebee wrote: »

hmm i think it warrants investigation

if a short circuit has bypassed the plug fuse and rcbo

he'll have to report back the PFC at the socket anyway to get a better idea

Live-Neutral the wrong way around feeding the RCBO possible? Then the plug fuse and the RCBO 20amp part of the breaker is also bypassed. In this case all the sockets on that circuit would show reverse polarity. I actually do think it warrents a check of the installation. Rcd part should still of tripped though if earth fault involved.

While discrimination may not work in large fault currents, you certainly would expect a 13 amp plug top fuse to go.

Its also hard to imagine a fault in a transformer winding giving a direct short L-N, And also hard to imagine a short in the transformer giving rise to any massive high fault current capable of frying an RCD. Short circuits do strange things though, i shorted a socket circuit by accident before and at the same instant a bulb blew out of its bayonet cap, the glass bit blew right out, how would that be explained?

2011

It would be doing exceptionally well to blow a distant 63amp neozed and leave the 13amp plug fuse intact though if it was the transformer that blew the main fuse.

But it did as the OP pointed out.

Look at the fusing characteristics for a very large current. Also how do you know that the fuse was not weakened previously from sustained overload?

Bruthal

2011 wrote: »

But it did as the OP pointed out.

Look at the fusing characteristics for a very large current. Also how do you know that the fuse was not weakened previously from sustained overload?

You reckon a short circuit will trip the 63 amp main fuse, and leave the 13 amp plugtop one intact, and also not trip the socket circuit 20 amp breaker? Its possible alright if the main fuse is already loaded near to its rating, but he said there was no load on it.
After all that you say the circuit does`t need to be checked? Maybe your right, but id certainly check polarity at least on sockets.

What sort of current over time would be required to weaken the 63 amp fuse so it now discriminates ahead of a 13amp plugtop fuse and the MCB.
Maybe the fuse was faulty already, if it was continously overloaded there will usually be overheating signs around it.

Maybe a faulty 63amp main fuse is the cause, its the simplest idea anyway.
Maybe it never happened at all.

Bruthal

2011 wrote: »

But it did as the OP pointed out.

Look at the fusing characteristics for a very large current. Also how do you know that the fuse was not weakened previously from sustained overload?

I think you might of missed my point on saying its hard to imagine a 13amp fuse not going when a 63amp one did, i am getting at a polarity reversal, therefor bypassing the 13amp fuse. Or else a very faulty 63 amp fuse.

We dont know anything about the installation, where the socket was actually fed from, nothing, all we can do is suggestions.

Bruthal

2011 wrote: »

But it did as the OP pointed out.

Also how do you know that the fuse was not weakened previously from sustained overload?

I dont, but i did suggest that the main fuse may have been loaded near capacity and the OP said he can guarantee there was no loading on it. Also its a new installation i thought he said. It one of them ones for Unsolved Mysteries.

2011

If you have reverse polarity the same current will flow through a fuse on the neutral as it would on the live, so it would make no difference.

In theory fuse discrimination works 100% of the time, in practice it is sometimes different.

Main fuses sometimes fail. That is life.

Perhaps the fuse almost blew in the past and was weakened. The short circuit could have been the straw that broke the camels back.

M cebee

2011 wrote: »

If you have reverse polarity the same current will flow through a fuse on the neutral as it would on the live, so it would make no difference.

In theory fuse discrimination works 100% of the time, in practice it is sometimes different.

Main fuses sometimes fail. That is life.

Perhaps the fuse almost blew in the past and was weakened. The short circuit could have been the straw that broke the camels back.

the plug fuse is bypassed for L-E short circuit

with steel conduit of course this fault current is increased

in theory anyhow

who knows maybe they're cheap breakers

Bruthal

2011 wrote: »

If you have reverse polarity the same current will flow through a fuse on the neutral as it would on the live, so it would make no difference.

In theory fuse discrimination works 100% of the time, in practice it is sometimes different.

Main fuses sometimes fail. That is life.

Perhaps the fuse almost blew in the past and was weakened. The short circuit could have been the straw that broke the camels back.

Well reverse polarity at a socket leaves the plug fuse out of the circuit in the event of a neutral earth short in the transformer, the neutral in the transformer is now the live terminal, if it shorts to earth then the fault current does not go through the fuse, thats why if an appliance trips an mcb or fuse but does not blow its plug fuse then socket polarity must be one check that is done. That was my whole point about how the 13amp fuse managed to stay intact.

And descrimination does not even come into this. Any competent sparks knows several devices in a line can operate in fault conditions, but how a 13amp plug fuse stays intact would have to be questioned, and you said yourself it was likely a high fault current flowed, and thats where the reverse polarity suggestion came in, its only a suggestion.

If the RCBO live-neutral supply are reversed then the MCB part of it is also left out of a neutral-earth short which is now a live-earth short,


And main fuses do indeed fail, and it could well be that, thats the simplest cause and often it is the simplest solution that is the cause.

What tester was used to check the transformer, some continuity testers will beep through a transformer windings as if it were a piece of copper, an ohms reading live to neutral on the transformer is whats needed to show its condition. Maybe there is nothing wrong with it and the RCBO itself shorted.

Bruthal

M cebee wrote: »

with steel conduit of course this fault current is increased

in theory anyhow

Id say in practice also,

M cebee

ya in practice

the steel in parallel with the cpc

gives you a lower Zs than equivalent 3-core


could be an issue if the rcbo can't break the fault current at the socket-

when you get back to the sub-board the fault current will be higher again and must be 'doubled' if sub-board is 3-phase

i would say the neozed shouldn't let through enough energy to fry the 'rcbo'

in theory anyhow -who knows strange things happen with electrics

Bruthal

M cebee wrote: »

ya in practice

the steel in parallel with the cpc

gives you a lower Zs than equivalent 3-core


could be an issue if the rcbo can't break the fault current at the socket-

when you get back to the sub-board the fault current will be higher again and must be 'doubled' if sub-board is 3-phase

i would say the neozed shouldn't let through enough energy to fry the 'rcbo'

in theory anyhow -who knows strange things happen with electrics

Its not just the neozed not letting enough energy through, the entire circuit has enough impedence to not let a massive current flow. If you just connect live to neutral at the back of a socket and turn on MCB you could reset it dozzens of times and the breaker will just keep tripping with that popping sound im sure we are all familiar with without any welding of contacts. The fault current gets larger for the same average conditions the nearer the installation is to the 10kv-400/230v transformer.

As i was saying, an ohms reading is needed of the 110v transformer L-N with a multimeter on the low ohms setting, it could just be a short at or around the RCBO in this circuit which would of given a nice high fault current, and the 110v traffo was not faulty at all. Its hard to imagine a fault in the trafo destroying the RCBO, and not touching the plug fuse except in socket polarity reversal.
And even if there was a complete short in the traffo, it still would`t be likely to destroy the RCBO. All the theories in the world can be mentioned about high currents etc, but in practice it seems unlikely, so id definitly agree its hard to see the energy flow required to fry the RCBO/contacts.

And id agree strange things happen with electrics also, like i mentioned shorting a socket L to N before and it popped the glass from a standard bulb right out of its BC, how does anyone explain that. If the RCBO was fried it would look like the short was around or in that, two flexible cable links shorted in DB or something like that.

slavetothegrind

just to clarify, there was no reversed polarity situation at either the socket or the rcbo, abb 16a by the way

Am going back there tomorrow so will grab the rcbo and investigate further.

Definitely the only load on the circuit. It is a 3 phase install but the unit is unoccupied at present and no other current was being drawn at the time apart from the lights which were on another phase.

i will check out the trafo further too and report readings across windings and to ground

M cebee

check the pfc with a loop tester

slavetothegrind

will do!;)

2011

the plug fuse is bypassed for L-E short circuit

Yes, fair point.

It sounds like you had a phase to earth or a phase to neutral shourt circuit. When this happened the fault current would have been very large. This large fault current could have fried your RCD.

When very large fault currents flow fuse discrimination may not always work the way you intend!

Nothing you have described would suggest to me that there is anything wrong with the wiring.

So perhaps the above is correct as the OP has confirmed that there is no reverse polarity.

Sometimes fuses fail. I have had ABB RCDs and RCBOs fail too when I have plugged 110 volt traffos in.

2011

Definitely the only load on the circuit. It is a 3 phase install but the unit is unoccupied at present and no other current was being drawn at the time apart from the lights which were on another phase.

Yes but the fuse may have been subjected to large currents in the past. It may have been on the point of failing and this issue pushed it over the edge.

Bruthal

2011 wrote: »

Yes, fair point.



So perhaps the above is correct as the OP has confirmed that there is no reverse polarity.

Sometimes fuses fail. I have had ABB RCDs and RCBOs fail too when I have plugged 110 volt traffos in.

No one is saying your incorrect, we were asked to explain how a 13 amp fuse in a transformer plug stayed intact while a distant 63amp fuse blew, and i gave the explanation how that happenes, and also suggested that maybe nothing is wrong with the traffo, and also suggested maybe the RCBO or link wiring around it shorted.

But no way can i ever see a 13 amp fuse survive a short circuit that blows a 63 amp fuse no matter what its condition. You seem determined about this 63 amp fuse being weakened, in reality the chance of that happening where a fault occurs and leaves the fuse operating while a 13amp plug one survives is very remote, but anything is possible. If you plug in a radio into a socket in a house and the main ESB fuse goes, but the radio plug fuse survives, would you not look at that as unusual? We are suggesting how a traffo plug fuse survived while a 63amp fuse blew, thats all, because thats what we were asked i thought.

If a large current did flow like you said and quoted yourself about here, then explain how the 13 amp fuse survived, i would be interested in that.

So what we have is one of the following...

A short in a transformer that blows a 63amp neozed fuse but not the 13 amp one. Possibilities? Reverse polarity bypassing the plug fuse is the first thing id look at there. After that its check is anything really wrong with traffo at all with an ohms test. Next posibility is a short in or arount the RCBO or its flexible tails in the DB. And the other possibility is a faulty 63amp fuse like you rightly put as a possibility, but its not the only possibility.

Now for a 63 amp fuse to be weakened or faulty so it blows and leaves a plug fuse intact seems unlikely,, its your old friend descrimination again, how low would a 63 amp fuse have to blow at to leave 13 amp fuse intact on a short curcuit?? Because of this i would of thought common sense would make a sparks wonder why? And is it not what the original question was....Explain this? And so i put forth suggestions,, now reverse polarity is ruled out i would go to the next most likely cause, that the fault was in the DB board, after all, a short N-L in traffo would of blown plug fuse, lets be realistic. The only scenario it would not blow the plug fuse is a reverse polarity neutral to earth short. But this should of tripped RCD part of RCBO, so now a short is looking likely in the DB and traffo is not faulty at all.

Maybe none of our suggestions are right, Its not a competition, just suggestions.

Bruthal

2011 wrote: »

Yes but the fuse may have been subjected to large currents in the past. It may have been on the point of failing and this issue pushed it over the edge.

It is possible alright, again no one is saying its not, but is this really the only thing that `should be considered?

You said you had RCDs etc fail when plugging in a 110v traffo, what were the nature of the failures? Dont forget we were told the 110v transformer had a short develop in it, we can only make suggestions and observations based on what we are told. I seen fellas put continuity testers on windings and because they beep`d they assumed a short circuit, But a lot of continuity testers will beep even on quite high resistances. An ohms reading is needed to show us.

Bruthal

M cebee wrote: »

check the pfc with a loop tester

Thats a good suggestion, although an ohms test of traffo might show us there was never anything wrong with the traffo, and the problem is simply a short in the DB. I seen 13amp socket internal copper strips short together before also.

2011

You said you had RCDs etc fail when plugging in a 110v traffo, what were the nature of the failures?

I have had it happen with RCBOs and RCDs. The traffo is plugged in, the device trips and it will not reset or it will and the test button no longer works. I would replace the RCD or RCBO and it would be fine again. Like I said happened mainly with ABB as far as I can recall.

The other thing that can weaken a main fuse and cause it to blow prematurely is if the fuse holder is not making a good connection with the fuse. This can cause the fuse to get hot. The heat can start to melt the fuse wire inside the fuse.

But no way can i ever see a 13 amp fuse survive a short circuit that blows a 63 amp fuse no matter what its condition.

No matter what the condition??? I would not agree. If the condition of a fuse is bad enough even vibrations can cause it to fail.

You seem determined about this 63 amp fuse being weakened

No I am offering it as a possibility. I have seen fuses of all sizes fuses fail and I have also seen discrimination not work.

Possibilities? Reverse polarity bypassing the plug fuse is the first thing id look at there.

Possible, but ruled out by the OP.

Next posibility is a short in or arount the RCBO or its flexible tails in the DB.

Perhaps, but that would mean that the timing was a coincidence. So although that is possible it would be a long shot.

the other possibility is a faulty 63amp fuse like you rightly put as a possibility, but its not the only possibility.

I may be wrong, but on the balance of probability it adds up to me including the sequence of events and the timing.

is this really the only thing that `should be considered?

Of course not. Solving electrical problems is a process of elimination. Start with what is most lightly to be the problem.

I seen fellas put continuity testers on windings and because they beep`d they assumed a short circuit, But a lot of continuity testers will beep even on quite high resistances.

This is true. In general transformers have a very low resistance as the windings are made of copper.

Bruthal

2011 wrote: »

I have had it happen with RCBOs and RCDs. The traffo is plugged in, the device trips and it will not reset or it will and the test button no longer works. I would replace the RCD or RCBO and it would be fine again. Like I said happened mainly with ABB as far as I can recall.

The other thing that can weaken a main fuse and cause it to blow prematurely is if the fuse holder is not making a good connection with the fuse. This can cause the fuse to get hot. The heat can start to melt the fuse wire inside the fuse.


No matter what the condition??? I would not agree. If the condition of a fuse is bad enough even vibrations can cause it to fail.


No I am offering it as a possibility. I have seen fuses of all sizes fuses fail and I have also seen discrimination not work.


Possible, but ruled out by the OP.


Perhaps, but that would mean that the timing was a coincidence. So although that is possible it would be a long shot.


I may be wrong, but on the balance of probability it adds up to me including the sequence of events and the timing.


Of course not. Solving electrical problems is a process of elimination. Start with what is most lightly to be the problem.


This is true. In general transformers have a very low resistance as the windings are made of copper.

O no not the multiple quote, well i remember being told the reverse polarity would make no difference, i guess your right.

2011

Robbie:

well i remember being told the reverse polarity would make no difference

Yes. My point was that the same current flows in the live as the neutral when there is a N to L short circuit. I forgot about the L to E short circuit until M cebee corrected me

M cebee:

the plug fuse is bypassed for L-E short circuit

My reply:

Yes, fair point.

Bruthal

2011 wrote: »

Robbie:
Yes. My point was that the same current flows in the live as the neutral when there is a N to L short circuit. I forgot about the L to E short circuit until M cebee corrected me

M cebee:

My reply:

Did you not read my post (#4) where it says about plug fuse bypassed by reverse polarity as a possible scenario and neutral to earth (now live to earth)would now bypass the fuse, ? And #2 where i suggested an already at limit main fuse?
Anyway you have suggested it was probably a large short circuit current that flowed, and as you now say its probably too much of a co-incidence for the short to be in the DB i assume you believe it is in the traffo, so now again, how did the 13 amp fuse escape the "large short circuit current"? The main fuse was so weak it went first? I think its likely the main fuse was new in a new installation, I think that is a very fair question i ask, and the OP also asked. Im not asking as a test, even though multiple quote answers to someone`s points are like a master correcting the pupil:), im just asking because im curious.

Anyway i must admit, if i had a traffo blow a main fuse while its plug fuse stays intact, the very first test i do after replacing the main fuse is a test of the socket the traffo was plugged into and of the traffo itself, thats just me, everyone else can assume it was a 90 percent blown fuse if they want to. The main fuse would of shown plenty of discolouration on the metal caps if it was subjected to being loose and overheating.

Of course a weak fuse is a possibility, its not rocket science, but when a main fuse discriminates over a plug fuse, surely some check of the socket in question should be considdered.