How easy is it to blow the DNO's cutout?

[Mark42]

I've been volunteering at my local village hall, installing audio and light systems. Converting what was originally an underused sports hall into a venue for bands and theatrical events.

As with most of my projects, it has got out of hand and I've now reached a load schedule of 56A for my lighting rig alone, nearly all of which is LED.

Of course there's diversity: never will all fixtures be at 100% intensity together, but adding the legacy overhead of about 50A (hand driers, kitchen kit, kettles, portable heaters) I worry that I may potentially be stressing the supply. It's only a mickey-mouse domestic single phase cutout, probably containing an 80A fuse.

In others' experience, how likely is LED inrush to blow the cutout? It would be unfortunate if a widespread strobe or flash lighting cue closed everything down by killing the building's power!

Of course careful programming can keep the load down, but it may not always be me on the Avolites desk. If we have some old-skool techno operator who goes bananas, there may be a problem.

I have wired everything on a new three-phase submain DB (with L1+L2+L3 temporarily commoned), and asked the committee to upgrade to 3 x 100A, but it will take years to get agreement, if ever.

 

[James]

I would start with a logging clamp meter and do some testing

 

[Avo Mk8]

The fusing characteristics of the main fuse means a huge surge is needed for it to blow instantaneously, and a higher sustained current than the rating of the fuse to blow it later. You'll get 100A forever from a 80A fused supply (well, until something else gives up or catches fire!). And just to take a figure at random from the graph, 300A for not more that 10 seconds.
Not that I'm recommending that!
But hopefully the equipment you have, and that brought by artists, will average out to the extent of not being a problem. As has been said, logging what's happening would probably give some peace of mind. Even a cheap clamp ammeter from Screwfix has a peak current display feature (except the instrument switches itself off after a while!), so you could keep an eye on what's happening, if you aren't already!

 

[Rockingit]

You'll only see a surge when you lamp on (old terminology!) and experience tells me that your LX load won't be anywhere near your calculation in practice. Easy way to check (as suggested above) is just to use a clamp meter - set all the movers to white and on a pan/tilt move chase and see what you're drawing. Assuming that the moles and strobes are also LED then you'll be fine.

However - personally I'd be more concerned about how much earth leakage all those cheap fixture are dropping and how you've managed that cascade over the correct A type RCD's.

ps.... Who's that rig been designed for? A resident Abba tribute??!

 

[oscar21]

I managed to blow a DNO fuse the other month, well it was my fuse but same thing. Some of the places we work in are fire damaged or flood damaged so half the time there isn't any power on when we get there, obviously people need to charge batteries, boil the kettle etc so I usually temp something on.

In this case someone had removed the cutout fuse and put the holder back into the carrier. I got a fuse out of the van, wired my temp supply and energised everything and it all worked absolutely fine all day. Came back in the next day and there wasn't any power on. Upon checking the main fuse (80A I think) had blown, no black marks anywhere, even the RCD and MCB was still on and it only supplied a couple of twin sockets anyway so couldn't have overloaded and whatever happened happened after we all left anyway.

So, I got another fuse out of the van, plugged it in and there was an almighty bang, sparks hit the ceiling and it felt like an eternity before the fuse blew, very strange, don't understand what went wrong, I thought maybe the meter had failed so disconnected that, put a very thin piece of wire across the fuse, one strand of copper flex cable I think, tried again and there was still a big bang again but not as violent.

Investigating further and the outgoing live from the cutout was shorted straight to the chassis of it, don't know how that could happen with no-one around at night but it did.

 

[littlespark]

Did you not test anything before energising?

 

[oscar21]

Did you not test anything before energising?

Would it have mattered? there wasn't anything to test really, does anyone check the cutout terminals to earth before they whack the main fuse in. Putting the main fuse back in is always an exciting moment at the best of times.

 

[westward10]

I wouldn't have even admitted to any of that.

 

[oscar21]

I wouldn't have even admitted to any of that.

For legal reasons or stupidity reasons? some people are under the impression that there are an electric police that go round lifting people that cut the seals. Some say that smart meters monitor when the power goes off and call the SAS the moment it happens, yes there is a micro switch under the cover but I had a smart meter sat on the floor for most of the day the other week whilst I rearranged everything inside the cupboard, no-one turned up, not even a traffic warden.

I've been in a couple of situations similar over the years, one all the meters were credit meters and I needed to do EICR's on several flats I had fuses/meters bridged out all over the show and someone turned up to read the meters, he said cheers mate and went on his way after I showed him where they was,, nothing ever came of it but I did find my mate hiding in the back of the van 10 minutes later.

Another example was a building contractor who bought a derelict building that had had the power cut off to it for lack of payment presumably. The meters were in the cellar which he wanted to convert, he knew an iffy DNO guy who he got to move the meters to the outside wall of the property so got it done on a Saturday morning. I went in the afternoon to fit the isolators and submains etc. I was sat there with my legs dangling into the (jointing) hole doing the isoaltors when a couple of guys from the metering company turned up to see how they could get the power back on after the builder had contacted them about re-energising the supply.

I said oh, I think it just needs the main fuses putting back into the cutouts and its ready to go, he said is all this new, I just shrugged my shoulders and said I dunno mate, I'm just here to connect the submains. And that was that, we had power again.

In situations like this I think the DNO and metering companies are well aware of "its not what you know but what you can prove" if they ever went to great expense to bring a prosecution, so unless they were to catch you red handed stealing electricity I don't think there is much they can do.

 

[westward10]

For legal reasons or stupidity reasons? some people are under the impression that there are an electric police that go round lifting people that cut the seals. Some say that smart meters monitor when the power goes off and call the SAS the moment it happens, yes there is a micro switch under the cover but I had a smart meter sat on the floor for most of the day the other week whilst I rearranged everything inside the cupboard, no-one turned up, not even a traffic warden.

I've been in a couple of situations similar over the years, one all the meters were credit meters and I needed to do EICR's on several flats I had fuses/meters bridged out all over the show and someone turned up to read the meters, he said cheers mate and went on his way after I showed him where they was,, nothing ever came of it but I did find my mate hiding in the back of the van 10 minutes later.

Another example was a building contractor who bought a derelict building that had had the power cut off to it for lack of payment presumably. The meters were in the cellar which he wanted to convert, he knew an iffy DNO guy who he got to move the meters to the outside wall of the property so got it done on a Saturday morning. I went in the afternoon to fit the isolators and submains etc. I was sat there with my legs dangling into the (jointing) hole doing the isoaltors when a couple of guys from the metering company turned up to see how they could get the power back on after the builder had contacted them about re-energising the supply.

I said oh, I think it just needs the main fuses putting back into the cutouts and its ready to go, he said is all this new, I just shrugged my shoulders and said I dunno mate, I'm just here to connect the submains. And that was that, we had power again.

In situations like this I think the DNO and metering companies are well aware of "its not what you know but what you can prove" if they ever went to great expense to bring a prosecution, so unless they were to catch you red handed stealing electricity I don't think there is much they can do.

For sheer stupidity.

 

[bill01803]

I always put a Drummond acroos the mains cut out before putting the fuse back in

 

[oscar21]

For sheer stupidity.

So what would you do? You get to site with half a dozen other lads who all have mortgages to pay and the job is due to be signed off it 4 weeks do you say hey tough luck lads, its the DNO that replaces that fuse not me but they should be along in a few weeks if you are all ok with that. or do you get some power on by hook or by crook so everyone still has a job?

The last two jobs I've turned up at they have had to get an extension lead from next door and pay the neighbour £20 a week.

 

[oscar21]

I always put a Drummond acroos the mains cut out before putting the fuse back in

Whats a drummond and would it have helped me?

 

[timhoward]

or do you get some power on by hook or by crook so everyone still has a job?

I'm going to give you a line from my old gaffer. He's dead now (from old age) and had a lifetime of electrical experience.
"Many people turn on and hope it won't go bang. We are electricians and we only energise if we know it won't go bang. It's less exciting but we live longer"

Whats a drummond and would it have helped me?

A test light. A 2 pole tester would do the same. In your case there would be a path from supply side through the tester to consumer side to earth. It would have lit up showing you were about to connect a dead short. You would go 'hang on a minute' (hopefully).

 

[littlespark]

It’s all “exciting” and “ gung ho” when things go bang…. But accidents do happen…

Why do the real DNO guys wear rubber gloves, safety goggles and a fully paid up life insurance policy.

Are you the same king of cretin that flies up the motorway at 100mph, just for the thrill?


This thread is in the public area of the forum, and you’re not painting a very professional picture of yourself at the moment.

 

[DPG]

I think more care should have been taken bearing in mind it was known the fuse had already blown once. And we all hopefully know how much current must have flowed for that to happen.

 

[oscar21]

It’s all “exciting” and “ gung ho” when things go bang…. But accidents do happen…

Why do the real DNO guys wear rubber gloves, safety goggles and a fully paid up life insurance policy.

Are you the same king of cretin that flies up the motorway at 100mph, just for the thrill?


This thread is in the public area of the forum, and you’re not painting a very professional picture of yourself at the moment.

Firstly I don't need to paint a professional picture of myself, I'm just an anonymous smuck on here, I won't lose any work over it. Secondly whats with the virtue signalling, preaching that anyone who does 100 mph is a cretin yet you more than likely do 30 past a school at kicking out time without a care in the world which is far more dangerous than 100 on a motorway.

I put a fuse in, it went bang, so what.

 

[oscar21]

I'm going to give you a line from my old gaffer. He's dead now (from old age) and had a lifetime of electrical experience.
"Many people turn on and hope it won't go bang. We are electricians and we only energise if we know it won't go bang. It's less exciting but we live longer"

A test light. A 2 pole tester would do the same. In your case there would be a path from supply side through the tester to consumer side to earth. It would have lit up showing you were about to connect a dead short. You would go 'hang on a minute' (hopefully).

And yet everyone I know has a story of a big bang that happened to them, I've never met an electrician yet that hasn't had quite a big shock at some stage despite being a professional, I'll bet you have, sometimes things go wrong and 99.9% of the time you live to tell the tale, ---- happens.

As for the tester, wouldn't it light up anyway with a path through the meter as it was still wired in at that stage. The fuse I replaced (twice) was out of one of those isolatotor units where you get a pack of three 60/80/100 and when it didn't work my initial reaction was it must have been a cheap chineese fake fuse that just stopped working, no other explanation really.

Can you explain how a perfectly working cutout suddenly developed an absolute dead short to ground overnight without anyone being anywhere near it. I had a prod around inside it to try and see and the top of the carrier still looked like it was sat on an insulator to me.

 

[westward10]

So what would you do? You get to site with half a dozen other lads who all have mortgages to pay and the job is due to be signed off it 4 weeks do you say hey tough luck lads, its the DNO that replaces that fuse not me but they should be along in a few weeks if you are all ok with that. or do you get some power on by hook or by crook so everyone still has a job?

The last two jobs I've turned up at they have had to get an extension lead from next door and pay the neighbour £20 a week.

Not my problem.
You have a ruptured service head fuse, your solution put some copper across it, result bang. Not exactly level headed thinking is it you don't sound a very safe person to be in close proximity to.
Pray tell me you don't actually train people.

 

[littlespark]

Instead of 100mph, i should have said "cretins, deliberately breaking the speed limit" just for the thrill of it.... and where on earth can you do 30mph past a school at chucking out time?
Not round here.... not with all the double parked Landrovers.