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FIRE SERVICE AND GENERAL FIRE SAFETY TOPICS => Fire Investigation => Topic started by: st185cs on March 23, 2008, 09:17:59 PM
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This is my first post so please be gentle with me!
Has anyone heard of this problem before? All the indicators are that the seat of the fire was in this area and the intensity of the fire burnt a hole in the backboard as well as causing other serious damage. The main cartridge fuse failed to disconnect, the other consumer unit trips failed to operate and the street lights were flickering (4.30am). The supply is an overhead type in a rural/wooded area on the outskirts of a village.
Any views would be received most gratefully.
Thank you in anticipation.
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Sounds to me like a possible fault with the neutral conductor- possible on the over head power line local to the incident. As in rural areas the "PME" system is used- the consumers earth is directly connected to the neutral- if the neutral fails all current return is via the earht conductors which in older installations will most likely not be able to handle the current and any dirty or high resistance connection in the earth system will get HOT, and thin earth connectors and any bonding will probably overheat.
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....All the indicators are that the seat of the fire was in this area and the intensity of the fire burnt a hole in the backboard as well as causing other serious damage. The main cartridge fuse failed to disconnect, the other consumer unit trips failed to operate....
The failure of anything to trip in the consumer unit strongly indicates the fault was upsteam (ie towards the supply from the unit).
The failure of the supplier's cartridge fuse to 'blow' indicates a high resistance fault rather than a direct short and would support kurnal's comments re a possible neutral fault. A high resistance fault would allow a substantial amount of heat to be generated but without the fuse blowing.
A check of adjacent premises might be worthwhile to see that they've not been affected by the fault which could have been external to the affected premises.
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Sounds to me like a possible fault with the neutral conductor- possible on the over head power line local to the incident. As in rural areas the "PME" system is used- the consumers earth is directly connected to the neutral- if the neutral fails all current return is via the earht conductors which in older installations will most likely not be able to handle the current and any dirty or high resistance connection in the earth system will get HOT, and thin earth connectors and any bonding will probably overheat.
Sorry to disagree but!
As the power lines are overhead it is highly unlikely to be PME earthing system (Protective Multiple Earthing) as its unlikely to be a utility company supplied earth, which in TT arrangements the properties earth is derived from a local earth rod. within a TT earthing system the Neutral will not be connectd to the consumers earth. this is now relatively unusual now as most electrical supplies are forms of PME where the earth is tied to neutral, however this is not as easy to provide from overhead power lines.
I do have agree that the fault may well have been a relatively high resistance fault which has overloaded the capacity of the supply fuse, eventually overheating the supply cables or connection to of from the cutout, could have been as simple as a lose connection of one of the cables (live or neutral) causing a overheating connection.
If the cause was a loose connection of the incoming connection to the cut-out then the fault current would increase until the cable failed or the utility companies device activates, the fault levels involved may cause significant volt-drop in the local supply so if the street light was on the same phase as the hosue then they could be related.
Do you have any photographs or can you say where the burn mark was in relation to the rest of the supply cut-out?
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Thanks wormhole I never mind being corrected.
I have seen this sort of fire a couple of times in my career, it sounds similar to one involving a row of 4 trerrace houses about 20 years ago in which a neutral fault in the feed to the row (the 3 phase overhead supply fixed to the gable of one house then one phase and a neutral into each property) caused multiple small fires simultaneously in two houses both around the electricity meter board and consumer unit and also in a central heating pump in one house and and burned out a washing machine or similar in another. I never did get a definitive reason for it from the elctricity supply authority but the engineer who attended said it was a neutral fault.
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Thanks wormhole I never mind being corrected.
I have seen this sort of fire a couple of times in my career, it sounds similar to one involving a row of 4 trerrace houses about 20 years ago in which a neutral fault in the feed to the row (the 3 phase overhead supply fixed to the gable of one house then one phase and a neutral into each property) caused multiple small fires simultaneously in two houses both around the electricity meter board and consumer unit and also in a central heating pump in one house and and burned out a washing machine or similar in another. I never did get a definitive reason for it from the elctricity supply authority but the engineer who attended said it was a neutral fault.
I suspect that in the previous occasion that the neural connection was lost in the overhead supply, this could cause the neutral voltage to 'float' increasing in potential above the usual 0V (well 50V max) - this would not be a fire hazard normally but in a TT installation there is no connection from neutral to earth, which would stop tthe rise in potential.
If the load was a balanced (load on each phase equal) then the load on the neutral connection would be 0, then no rise in the neutral voltage.
Without the neutal connection the voltage between the phase and the neutral connection will move around, causng higher or lower differance in potential, the lower voltage differance can cause some appliances to fail as motors burn out, some applicances could cause fires as the measured supply voltage would be much higher than normal. - this would explain the differnet efffects in the different houses, although it would be possible to suffer damage from under and over voltage in the same house as the load in each house varies causing the neutral point to move.
Well thats the only thing I can think of anyway!
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Been looking at my notes of that incident. I used to keep my fire investigation notes in case the job ever led to a court case.
The two houses affected by fire damage ( smoke and scorching rather than flames) were on the same phase. The pole at the top of the field leading down to the houses had a small white plate sayng "PME system- do not disconnect neutral"
Notes refer to photos but sadly I dont have those. Photos were taken of charring of wood and overheated terminals on earth wires on main electrical board in both houses, central heating time switch burned out in one house, and next day caller of one house phoned to say washing machine which was on at the time was found to be defective once supply had been returned to normal.
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If the neutral connection was lost between the supply transformer and the distribution cable, on a polyphase system, you would end up supplying the line voltage (400 v) between each of the loads on the three phases, This would cause most equipment to stop working......at best. Actual voltage would depend on the circuit load.
If the earth electrode connection was lost at the local pole (Usually PME marked), then it is possible that there would be a potential difference between the PME'd earth / neutral connection and the local earth. If the main bonding within the installation was carried out correctly, this would reduce the risk of electric shock, but could increase the risk of heating / fire if a phase-earth fault occured upstream and the bonded services (Structural steelwork / gas / water supplies) provided a lower resistance route to the star point of the distribution transformer than the supply neutral cable. Which unless you live next door to the transformer, or live in an area with permanently saturated ground is a near impossibility.
Paul
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Dear all
Thanks for all the info and apologies for not getting back sooner (work kept me away!)
wornhole asked if there are any photographs? Yes, and I will email some shortly as I don't yet know how/if I can post them on here.
One photo shows one of two meters which were present. On the left there was another meter with the main cartridge fuse below (I think). It is clear to see where a hole has burnt straight through the company base board where the second meter had been.
The reason the meter and fuse holder were missing is due to the Brigade removing them (for inspection I guess?) and there was no plastic left on the meter whatsoever and only the metal components "survived".
Before I enquire, does anyone know if the Brigade investigators are able to release their report? I'm also wondering if any recent changes in how the building is used and heated could have been a contributory factor if they consumed much more power than before? I guess the rcd on the consumer unit would have tripped first in an overload situation
Thanks
Mark
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Additionally, there are only two overhead cables.
One is insulated, one is not.
On the neutral cable there remains a short length of temporary sleeving left from a time before which has melted and is scorched near where it has come into contact with the bracket on the outside of the building. The two cables had become twisted in two areas between the house and the pole but there were no signs that the insulation on the live cable had broken down.
The street light is on the next pole upstream and comes direct off the single live and neutral cables.
The network company have replaced the cables from the pole to the house in the last couple of days.
The next door neighbour (150+ yds away) had to reset their clocks the next day but hadn't noticed any component failures. The neighbour noted that they are always losing power.
I have also just learnt that an additional number of night storage heaters had recently been added to the house system and very recently the same (NIC) electrician found that the system earth had only relied upon connections to water pipes for some years so has had to fit three earth rods and an earth mat to get an acceptable reading.
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you shouldnt get any surges or problems from the incoming supply
The isolators on the pole should kick in, and as a last ditch line of defence the consumer unit should stop any harful faults surely
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Yes the fire service have a duty to investigate fires and if you are the owner of the property you are entiltled to be informed of their findings. You should contact them in writing and formally request this. They may charge you if you want a copy of the full report though. Write to the Chief Fire Officer at their brigade HQ.
Or perhaps ring their brigade control room and ask for the investigation offier to call you to discuss the incident.
Are you claiming on your insurance? The insurance companies have a team of expert consultants and may be interested in finding out what happened to see if anyone was liable.
How were the two meters connected to the incoming supply?
Was there a junction box after the cartridge fuse or did both sets of meter tails connect directly into the cartridge fuse?
Why were there two meters and what did they supply?
Were both meters the property of the electricity supply company or was one a private meter say for a shed or flat?
What was the rating of the cartridge fuse and did it blow?
Are the two areas of fire damage on the board just local to the meter and the fuse or is there any chance direct burning of the wood if either one had overheated may have set the other on fire?
Is the depth of fire damage to the wood deeper next to the fuse or next to the meter?
If both areas of damage to the wood are localised then it could possibly indicate two faults, one only becoming apparent because of the other - a partial short in the meter and a poor connection on the fuse limiting the current that can flow in a fault condition but allowing sufficient current for normal use, the extra current due to fault conditions causing arcing .
As the fire occurred at 4-30 am then those storage heaters would have been at full belt and perhaps a bad connection in or to the meter may have caused arcing? This may be the most plausible explanation.
NB this is all pure speculation and intended only to assist you to think this through. I am speaking as a fireman and not as an electrician.
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I do have to say that having read this thread and the investigations I have personally carried out on consumer units / power company supplies, unless the cause is obvious, I have always sought expert electrical engineer advice to accompany the final report. Far from being sarcastic, I am impressed with Wormhole & Kurnels knowledge on the subject, (did I miss a module at Moreton or was I at Ugly Bugs?), seriously though, I write this as my knowledge of electrical circuits is basic and if challenged I could not defend my opinion sufficiently - am I alone on this?
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Sounds to me like a possible fault with the neutral conductor- possible on the over head power line local to the incident. As in rural areas the "PME" system is used- the consumers earth is directly connected to the neutral- if the neutral fails all current return is via the earht conductors which in older installations will most likely not be able to handle the current and any dirty or high resistance connection in the earth system will get HOT, and thin earth connectors and any bonding will probably overheat.
Sorry to disagree but!
As the power lines are overhead it is highly unlikely to be PME earthing system (Protective Multiple Earthing) as its unlikely to be a utility company supplied earth, which in TT arrangements the properties earth is derived from a local earth rod. within a TT earthing system the Neutral will not be connectd to the consumers earth. this is now relatively unusual now as most electrical supplies are forms of PME where the earth is tied to neutral, however this is not as easy to provide from overhead power lines.
I do have agree that the fault may well have been a relatively high resistance fault which has overloaded the capacity of the supply fuse, eventually overheating the supply cables or connection to of from the cutout, could have been as simple as a lose connection of one of the cables (live or neutral) causing a overheating connection.
If the cause was a loose connection of the incoming connection to the cut-out then the fault current would increase until the cable failed or the utility companies device activates, the fault levels involved may cause significant volt-drop in the local supply so if the street light was on the same phase as the hosue then they could be related.
Do you have any photographs or can you say where the burn mark was in relation to the rest of the supply cut-out?
You have commited a fatal error there and will have to shut down
Earth would be provided by the utility company as it acts as earth back to the main grid.
A localised grounding point would be provided by the house builder or electrical contractor which would disipate the electricity in the event of leakage within the property
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Hi Cleveland
Dont agree with you there, I think wormhole knows his stuff.
As we all know there are many ways to skin a cat and there are a lot of places wired and connected fifty and sixty years ago where standards are not as they should be and if the electricity supply company was asked to connect to them today would refuse to do so.
In terms of new property you are right and the ressponsibility lies with the builder to insall a safe system which will be checked by the supply company before they connect.
In the fire incident I was referring to ( Which is no longer relevant to the original posters query because the installations are so different) there was an earth connection to the cold water, gas pipes and central heating and that was about your lot. As both you and wormhole point out in a correct modern installation a good earth would be the responsibility of the builder in these cases.
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Oh sorry I misunderstood your previous posts i think.
How old is the property in question then?
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You have commited a fatal error there and will have to shut down
Earth would be provided by the utility company as it acts as earth back to the main grid.
A localised grounding point would be provided by the house builder or electrical contractor which would disipate the electricity in the event of leakage within the property
Hi Cleveland,
Sorry for the delay responding - busy weekend!
There is no legal or otherwise requirement for the utility company to provide an earth point for an installation - this is the responsibility of the consumer. More often than not the earthing forms a TN-C/TN-C-S type arrangement where at some point the earth and neutral connection are combined (the C stands for combined)
However depending on the local supply network they can also provide TT earthing arrangements where the neutral and earth are not combined, in these cases the consumer must provide their own earth (normally via a rod, rods or earth mat) - in these cases the earth MUST NOT be connected to neutral. On new TT installation (or most modifications) the supply must be protected by a RCD to reduce risk of fire amonst other reasons.
On a TT system there is no link between the supply authorities earth and the consumers earth.
you shouldnt get any surges or problems from the incoming supply
The isolators on the pole should kick in, and as a last ditch line of defence the consumer unit should stop any harful faults surely
There will be quite alot of surges and problems with the incoming supply, this is particularly bad where overhead lines are used - again its the consumers responsibility really, although the utility company has limits alot of surges and harmonics are cause by other consumers so very difficult to police.
The isolators on the pole are often just that, providing no protection, often just manual disconnection. protective devices at medium voltage are very expensive and its quite unusual to be installed on a pole.
I'm hoping for some emailed photo's off st185cs, then we can get back onto the subject at hand!
Kernal - Thanks :)
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There is no legal or otherwise requirement for the utility company to provide an earth point for an installation - this is the responsibility of the consumer. More often than not the earthing forms a TN-C/TN-C-S type arrangement where at some point the earth and neutral connection are combined (the C stands for combined)
However depending on the local supply network they can also provide TT earthing arrangements where the neutral and earth are not combined, in these cases the consumer must provide their own earth (normally via a rod, rods or earth mat) - in these cases the earth MUST NOT be connected to neutral. On new TT installation (or most modifications) the supply must be protected by a RCD to reduce risk of fire amonst other reasons.
On a TT system there is no link between the supply authorities earth and the consumers earth.
Yes that is what I was eluding to albeit in a very crude fashion.
There will be quite alot of surges and problems with the incoming supply, this is particularly bad where overhead lines are used - again its the consumers responsibility really, although the utility company has limits alot of surges and harmonics are cause by other consumers so very difficult to police.
The isolators on the pole are often just that, providing no protection, often just manual disconnection. protective devices at medium voltage are very expensive and its quite unusual to be installed on a pole.
Indeed but if there is a huge surge which could potential damage consumer units I understand there is equipment in place on certain poles to prevent this which is what I was on about. However having read Kurnals post again it seems he wasn't talking necessarily about power poles
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Indeed but if there is a huge surge which could potential damage consumer units I understand there is equipment in place on certain poles to prevent this which is what I was on about. However having read Kurnals post again it seems he wasn't talking necessarily about power poles
the protective devices on the poles are relatively primitive, surge protection is expensive and not normally installed on poles, while the utility will have some surge protection its quite limited and mainly to protect their network.
Even the limited surge protection will not protect from lightning strikes, which cause alot of damage to electrical equipment, especially electronic equipment.
A simple example of this we are currently working on - 11kV on poles for several miles, no surge protection, into Network Operator switchgear, into private 11kV switch gear, run underground to transformer - output of transformer connected to a panel board with some surge protection built in - however this surge protection is quite basic (although it costs nearly £1,000) and is not required by any statutory documents.
As it is, we should hopefully not have any serious surges, however a lightning strike to any of the network in the area (which as its overhead is quite likely) could create a fault large enough to destroy the surge protection, and other equipment.
the main problem is this type of voltage distortion is more often than not, either caused by lightning or another consumer.
think of the domestic arrangement, I could cause a fault in my house, causing a power surge, this would likely affect and could damage all the houses on this phase electrically on the street, as we are all connected directly to the same cable.
It can become a real nightmare
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well im blown away there
i always thought there was more protection than that. You learn something new everyday and my training on electricity supplies I was given as an operational officer was obviously very incorrect.
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Hence my post earlier on in this thread.......fire investigators have limitations too, seek advice where appropriate, we are not all electrical engineers