BANG!

[Neil WyattModerator @neilwyatt]

[Neil WyattModerator @neilwyatt]

I picked up a four-way plug strip earlier and there was a huge band and big white flash. It tripped the workshop ring breaker and the supply breaker in the house but not two RCDs in the path.

On dismantling the strip there was no obvious fault, but I could see where a small bit of metal must have got inside and shorted across the brass strips carrying live and neutral – no earth imbalance to trip the RCDs.

Whatever it was that cause the short vaporised, along with some of the brass and plastic.

I binned it, although it would probably have been OK, as at the far end to the problem was a crack and a hole about 6mm across (where the metal got in?)

I suppose what is striking is how bright it must have been to make such a flash visible through the casing, it must have been in the order of several tens of amps to trip the main breaker for the workshop spur instantly.

Neil

[Frances IoMParticipant @francesiom58905]

was the strip not fused – preferably with a 10A fuse as the usual cable used must be to Chinese standards.

[BazyleParticipant @bazyle]

n opportune moment to mention a system we have going at the men's shed. A local group collects things like that lead and similar scrap and have a team of people with various mobility problems who strip out the metal. This makes it higher value scrap and gives the guys inolved some occupational therapy. See if you can find a similar group near you.

[Steve Skelton 1Participant @steveskelton1]

Neil, potential fault currents can be in the order of thousands of amps depending on the impedence of the cables supplying it.

In an extension socket lead, I would imagine it is somewhere between 500 and 2000A depending on how close to the origin of the installation the socket was.

Steve

[SillyOldDufferModerator @sillyoldduffer]

Spectacular flash when a ribbon of aluminium swarf found it's way into my mini-lathe's control box and shorted out across the motor feed.

As it didn't damage the board or blow a fuse I reckon half the light was chemical – the swarf caught fire and went off like a photoflash.

I wonder how many of us have blown the tip off a screwdriver with a 300V 16uF valve power supply capacitor? Happy days…

Dave

[Nick Clarke 3Participant @nickclarke3]

Posted by SillyOldDuffer on 22/11/2020 17:57:26:

Spectacular flash when a ribbon of aluminium swarf found it's way into my mini-lathe's control box and shorted out across the motor feed.

As it didn't damage the board or blow a fuse I reckon half the light was chemical – the swarf caught fire and went off like a photoflash.

I wonder how many of us have blown the tip off a screwdriver with a 300V 16uF valve power supply capacitor? Happy days…

Dave

In the days when you could smoke in pubs panatella cigars came in tins about 90mm dia and 150mm tall litho'ed a beige/pink colour. When empty these were prized for workshop storage but the print soon wore off.

I was handed one with shiny metal showing in several places and asked if I could get the lid off as it was stuck.

Electric shock and tin thrown across the room with much swearing. It had insulating tape around the rim to insulate the tin from the lid and a large fully charged electrolytic soldered between the two! He only held the tin, not the lid while I grabbed one in each hand.

I don't know what the guy charged it from, and you can't get the tins today, thank goodness!

[Nigel BennettParticipant @nigelbennett69913]

I had a near similar experience once when I casually picked up a 4-way adaptor and the outer plastic shell crumbled in my hand. How I didn't short two contacts I know not – but I checked all the others in the house immediately after. I literally crushed the adaptor into hundreds of pieces with my hands as I threw it away. It hadn't been out in the sun or anything – it was tucked away behind the PC.

Some Chinese rubbish we'd got from a cheapie shop somewhere.

If you are thinking of doing the crush test on one, do unplug it first…

[Robert Atkinson 2Participant @robertatkinson2]

Another example of why VFDs shouls always be in an enclosure, swarf gets everywhere

[SamsarandaParticipant @samsaranda]

A few years ago now I was installing a new kitchen and drilled into a wall on which to hang a cupboard unit, there was an almighty flash and a bang, our cat who had been watching from a distance shot off upstairs. It transpired that I had drilled into a hidden and unforeseen spur of the downstairs ring main, the masonry drill had gone dead centre between the live and neutral, this promptly vaporised it and on subsequent examination I found that the episode had blown the providers main fuse, at that time I think it was 100 amps. We had recently moved into the property and the distribution board was about the most basic you could get and had rewireable fuses, needless to say that board is long gone and replaced with a split load board with mcb’s and rcd. My workshop has a separate board fed via an rcd and has a mixture of cartridge fuses and mcb’s, replacing the cartridges with mcb’s as and when I can get the correct ones. You can never be too careful when electrical safety is concerned, had quite a few incidences involving electricity and so far survived them.
Dave W

[simondavies3Participant @simondavies3]

Many years ago I was trying to fault find a failing power connection which I traced back to the fused consumer unit. Checking using my newly acquired digital voltmeter across the switch revealed that apparently 220V existed but nothing was happening at the business end of the circuit. I then switched to the 10A setting and placed the meter in line which showed a few milliAmps were flowing. After digesting this a bit more I decided to check the volts again. Unfortunately I did this before returning the meter to a voltage configuration and promptly put a straight short across 220 V – the loud bang and flash left me stunned for a while, although fortunately neither I nor the house was harmed. The meter apparently was not reacting though since shorting the leads or connecting one directly into the other socket gave no visible readings. It turned out that both leads had gone open circuit, swiftly cured by a trip to Maplins.

Ever after, I have made a conscious effort to take a current reading and then return the leads to the voltage configuration.

I think the flash was my guardian angel briefly attending to me…..

[Howard LewisParticipant @howardlewis46836]

Whist spending a year of my Apprenticeship in the Electronics and Vibration Lab, At the end of the day, I went to pull the plug for the Mullard Valve Tester from the socket. The top came off and finger and thumb went neatly across the Live and Neutral pins. Much to the amusement of my colleagues! Fortunately the flloor was proof against 5 Kv, so the shock did minimal damage, except to my pride.

More recently, one New Years Eve afternoon, the motor on the Myford emitted a big flash and ceased operating. Fortunately, the local motor rewinders still had a couple of chaps working. They soon diagnosed a bit of Ali swarf in the centrifugal switch, cleaned up everything and it worked again. Since the job was so quick, it had not even been booked in, So a drink for their efforts, and return home to refit the motor.

The next job was to make a simple cover to keep motor separate from stray swarf!

Howard

[Neil WyattModerator @neilwyatt]

Posted by Frances IoM on 22/11/2020 16:57:53:
was the strip not fused – preferably with a 10A fuse as the usual cable used must be to Chinese standards.

I didn't bother testing the plug fuse, I assume it blew too. It wasn't a cheap unit, just had been in the workshop for a decade or more.

Neil

[John BaguleyParticipant @johnbaguley78655]

My cordless kettle did a similar thing last week. I switched it on to make a cuppa, there was an almighty bang and it leapt off the base! It tripped the 32 amp breaker but the 13amp fuse in the plug was still intact when I checked it. Needless to say, the kettle went in the bin as it was only a cheapy and I ordered a new kettle.

Before binning it I took it to bits and there was no obvious sign of any damage. When I checked the element though it was high resistance between live and neutral and high resistance live and neutral to earth. Presumably the element had failed and shorted out inside.

John

[Harry WilkesParticipant @harrywilkes58467]

My biggest 'bang' was during my time in the steel works and it was a case of ' familiarity breeds competent' myself and a fellow sparky had just changed large DC compound motor normal procedure was to run the motor with the shunt field disconnected to check direction then the shunt field was connected and the series field shorted out. At the rear of the control panel the armature cables were red and the series field were both green so without checking I shorted out the two greens restored power I then signaled to the other sparky to try the motor when he did there was an almighty BANG filling the control room with dust. When the control room was clear we checked what the cause of the problem was I had shorted the armature connections due to the colors being reversed the 350 amp fuses had completely disintegrated so too the ceramic fuse carriers strangely the two fuse bases had survived !

H

[David Davies 8Participant @daviddavies8]

Simon 0362 wrote:-

"Unfortunately I did this before returning the meter to a voltage configuration and promptly put a straight short across 220 V."

This mistake is easily made. For this reason, at work, the leads supplied with (Fluke) multimeters were removed and replaced with leads having fused prods before the instruments were issued to our electricians.

Because of the fused leads an instruction was also made that the electricians should test their meters on a known live circuit at the start of each shift thus proving that the fuses within the prods of the leads were healthy.

Dave

[Martin KyteParticipant @martinkyte99762]

Apparantly AVO 8's are indestructable. Apart from,that is, when connected accross the Southern Region in current mode.

Not done by me I hast'n to add.

regards Martin

[Nigel Graham 2Participant @nigelgraham2]

Among my illustrious c.v. details are Post Office Telephones – Trainee Technician (Apprentice) – failed. One afternoon when I was seconded to the team installing the telephone system in the newly-built local Council Offices, I was asked to plug in a 13A extension-lead in an adjoining room. As I did so there was sharp crack, accompanied by a sheet of flame from between two fingers, and a scream I think was probably me, but which brought the others rushing to my aid. One took me back to the depot, with a strip of skin hanging from one finger, to patch me up and cure the non-electrical shock with a mug of tea. I expect they had an accident-book but nowadays we'd be filling in forms while drinking the tea….

I spent some years working with piezo-electric devices, and the larger used big p-e ceramic stacks in parallel. Those had to be kept shorted because if left open-circuit, even just the tiny mechanical shocks from moving them about, or from the daily room-temperature changes, were enough for them to build up dangerously high static charges. They caught me out once or twice, but luckily only by small warning "bites".

[Oven ManParticipant @ovenman]

Carbon fibre filaments can create some excitement among electrical gear. It took some little time to fathom out what was blowing fuses on the furnaces we had a Rolls-Royce for making carbon fibre back in the 70s. You can't see them coming and they don't leave much of a trace when they short out two phases. No burn marks, it all goes up as CO2.

Peter

[John BaguleyParticipant @johnbaguley78655]

You've just reminded me Nigel. I also worked for the GPO and then later BT. I was working in Belper telephone exchange installing another shelf of equipment on one of the old Strowger racks which involved fitting a new fuse mounting on the live busbars that ran down one side of the rack. It had to be done live to avoid interuptions to service. I had done this many times before but this time somehow managed to stick my 81's (pliers) straight across the busbars whilst wiring up the fuse mounting. This did cause an almighty flash and bang and took the main fuse out that fed that particular suite of equipment. Can't remember now but probably 250amp. Needless to say, my 81's were now missing a big chunk and two of my fingers got copper plated! Extremely painful and the pain lasted a couple of days. Fortunately, the chap I was working with managed to get a spare fuse in before anyone noticed the loss of service and the incident went unreported!

Apparently, a chap doing some painting in Derby exchange put his tin of paint on top of the main busbars feeding a whole floor of equipment and that had quite spectacular results!

John

[Don CoxParticipant @doncox80133]

I too worked for Post Office telephones starting as a "Youth in Training." After a couple of enjoyable years charging about in a dark green Mini Van fixing customer's faults I was taken on as an exchange maintainer working on Strowger (clock work) equipment.

The contact array, which two motion selectors stepped over to make the forward connection, consisted of 11 contacts arranged in a semi-circle, stacked 20 high with plastic separators insulating adjacent contacts, these were known as "banks". Occasionally we would get a contact fault between contacts and since these banks were usually connected in multiples of 20 ( on two shelves of equipment) finding which bank unit was involved was quite a task. One of the old hands told me that these faults were usually caused by wayward bits of solder etc left behind during manufacture and that actually taking the trouble to locate the cause took up too much time and effort. The standard method to clear the fault was to apply the exchange 50 volt DC supply either side of the fault protected with the smallest fuse possible (0.25 Amps I think). Nine times out of ten this cleared the fault with no recurrence and didn't blow the fuse. The one in ten which persisted involve a lot more work, but that's another story. I have used this technique quite a few times to clear car electrical faults over the years too.

[GeorgineerParticipant @georgineer]

Photos or it didn't happen!

It was my favourite screwdriver too.

George B.

[Perko7Participant @perko7]

I have built a lot of hand-laid N-gauge model railway track using copper-clad sleepers. The sleepers are all gapped to prevent shorts but sometimes there are stray bits of copper bridging the gap which creates a big headache when fault-finding. I had an old Triang controller with 15V AC output rated for 2A with a relatively slow-acting thermal overload. I used to hook that up to each section of track and turn it on. Any stray strands of copper didn't last long.

[NealebParticipant @nealeb]

Due to crass stupidity on my part (I'll say it before someone else does…) I managed to put a bolt through the mains feed into my CNC control box when reassembling after maintenance recently. It took out the 32A ring main mcb plus upstream rcd in the consumer unit but two 13A fuses in plugs were untouched. So, if you want to avoid blowing fuses, just fit an MCB…

This episode actually shows how poor fuses can be for protection, and how people who talk about "protecting equipment by fitting a fuse" are often deluding themselves – fuses, and better still MCBs, are to stop the wiring bursting into flames after the equipment had already failed catastrophically!

[Steve Skelton 1Participant @steveskelton1]

Nealeb, fuses are probably the safest of all the circuit protection devices as they are fail-safe as long as they have not been interfered with. If they are from a reliable supplier they will fail according to a standard.

MCB's on the other hand may or may not always work to the standard to which they were produced for a number of reasons. Age and environment (temperature and damp for example) being the main cause of unpredictability. If an MCB rated at 3K breaking capacity has been exposed to a dead short which has produced a greater fault current than which it was designed to then it is likely to be permanently damaged and will probably not disconnect correctly, if at all.

RCD's and RCBO's have a high failure rate which is why they have a test button on them – it is not for show and should regularly be tested as it may offer no protection from stray residual currents. We have regularly found RCD's and RCBO's that have failed out of the box.

So the statement " shows how poor fuses can be for protection" is totally incorrect. MCB's are not as reliable as fuses which is why they are still used in such numbers in industrial situations.

You are right, however, when you say that the fuse/mcb is there purely to protect the installation (wires).

Edited By Steve Skelton 1 on 24/11/2020 12:54:08

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