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.

 

[oscar21]

Yes. I've taken out a supplier fuse when an entire installation was reverse polarity, and certainly mistakes were made leading up to this, but I learned from it.
I get that you left things working one evening, and the next morning for no apparent reason everything was dead.
Red flag number 1 was obviously that supplier fuse had blown. That would immediately make me think something very significant has happened.
Red flag number 2 was that the RCD and MCB were still in the ON position. So the cause must be upstream of them. This would put me in "something exceptionally unique and wacky has happened" mode and caution levels would rise to paranoid levels.

The absolute last thing I'd do is bung another fuse straight in and 'see what happened'. Continuity/IR testing at your incomer on your CU would be where my mind when next.
As for creating your own low value fuse, still without having any idea what caused the fault or finding a way to measure it, well I'm honestly rather stunned.

Out of interest, what was causing the short, the end of the tails, or the terminal itself?
Was the cut-out safe for continued use afterwards?

Yes there was a red flag moment but think about it, it worked fine, its probably worked fine since the 50's, yes the house had had a fire but it didn't get to the mains cupboard, the old plastic CU was still intact. So what could cause a 80A fuse to blow in the middle of the night with no-one around and absolutely no load on it, its not like the cut out has moving parts that had failed.

My conclusion to the red flag moment was it has to be a faulty fuse, so I replaced it. As for the very thin fuse wire trial, I thought that was quite a good idea, couldn't have been more than an amps worth of wire, I disconnected everything, even the tail out of the top of the fuse holder and it still went with a bang but with a lot less sparks and arcing this time.

s for the problem, I have no idea, I still cant fathom what went wrong, I got the office to call the DNO out and I wrote on the cut out "do not energise" By the time I came back to second fix it had a brand new head on it.


I still don't get all the hoo-ha about it though, it seems some people are ---- scarred of a big bang, you see it when someone is going to cut a cable which might be live with insulated snips, try as they might, they just can't bring themselves to do it even though they know they are perfectly safe from injury, its the bang you see.

 

[pc1966]

I still don't get all the hoo-ha about it though, it seems some people are ---- scarred of a big bang

You might want to look up arc-flash injuries. Actually, you don't.

The stats I remember from the USA was that about 80% of deaths suffered by electricians were due to burns, not shock. Now the situation UK versus USA is not quite identical as the USA has more issues of arc-flash for various reasons, but if you have seen what a high energy fault can do on a test rig, etc, you REALLY do not want to be part of it.

 

[littlespark]

If you make a big bang like that in full earshot of plumbers, joiners, builders… you’ll either

A- get the p ss ripped out for months on end
B- no one wants you back on a job because even they think it was dangerous.

 

[pc1966]

Sound is not quite in sync (earlier than video) due to video compression delay artefacts on cheap dashcam, but here is an illustration of why you wear PPE if working on live power circuits:

 

[James]

I am not sure that you realise the damage that closing a fuse on to a fault can cause.
on this occasion it seems like you were lucky.
these things can end in loss of life, if your lucky it’s just loss of face (sometimes literal)
my first thoughts are that you are an idiot, after further thought I think you are more educated than the average idiot however have little concern about the effects of your actions and are therefore just a risk taker.
good thing you have found employment because if you start telling prospective employers about your antics they might turn you down.

 

[oscar21]

I am not sure that you realise the damage that closing a fuse on to a fault can cause.
on this occasion it seems like you were lucky.
these things can end in loss of life, if your lucky it’s just loss of face (sometimes literal)
my first thoughts are that you are an idiot, after further thought I think you are more educated than the average idiot however have little concern about the effects of your actions and are therefore just a risk taker.
good thing you have found employment because if you start telling prospective employers about your antics they might turn you down.

I really do think most people have never worked on smaller informal sites that don't have the hi-viz brigade looking after them. You wouldn't believe what goes on regarding safety or lack of it. The labourer fell though some joists the other day right up to his -------s and brought a ceiling down with him, did anyone get upset? of course not.

I must live in a different universe to most people, just the other day (admittedly working at home) me and a mate got my RSJ steel into position by lifting it up a block at a time from either end until it was about two meters high, wobbled all over the show but it saved £100 on a genie, it wasn't even me that wanted to do it that way but I got called soft for not just getting on with it.

 

[timhoward]

Sound is not quite in sync (earlier than video) due to video compression delay artefacts on cheap dashcam, but here is an illustration of why you wear PPE if working on live power circuits:

I was looking for that video earlier, but couldn't find it.
There's another one where there is no way the person standing where the fireball happened survived, and it's simply too horrific to share a link to.

I really do think most people have never worked on smaller informal sites that don't have the hi-viz brigade looking after them.

This is the last place you want to generalise, we have forum members doing just about everything possible! I'm a sole trader and rarely do site work.

So what could cause a 80A fuse to blow in the middle of the night with no-one around and absolutely no load on it, its not like the cut out has moving parts that had failed.

So far my best wild guess is that things got marginally disturbed when the temporary supply was hooked up, and some kind of significant vibration occurred causing a further minor shift in either the terminal or the tails and BANG.

The bottom line is that a closing on a dead short or even a moderate load load holding an 80A fuse in your hand to complete the circuit is just damned dangerous, and the extent you are arguing against this basic fact leads me to believe you have no intention of ever taking it on board. I don't have anything further to add so won't be debating this any further.

 

[plugsandsparks]

I think in this case, the guy needs more experience, which i think he will get soon enough. He never did answer the question of what size fuse would he baulk at energising without IR testing first, still, cannot fault the attitude, like the guy jumping out of a 30 storey building...... so far so good, lol

 

[oscar21]

I think in this case, the guy needs more experience, which i think he will get soon enough. He never did answer the question of what size fuse would he baulk at energising without IR testing first, still, cannot fault the attitude, like the guy jumping out of a 30 storey building...... so far so good, lol

More experience, I'm 52 and have had more than most.

Its no good asking this question, how can you test a cut out before you put the fuse back in, I'll pull plenty of fuses out over the next months and years an I'll put plenty back in again, I'm not about to do a full forensic on every little thing I do from now on on the off chance that a 0.00001% fault will occur. I'd rather take my chances.

 

[oscar21]

I was looking for that video earlier, but couldn't find it.
There's another one where there is no way the person standing where the fireball happened survived, and it's simply too horrific to share a link to.


This is the last place you want to generalise, we have forum members doing just about everything possible! I'm a sole trader and rarely do site work.

So far my best wild guess is that things got marginally disturbed when the temporary supply was hooked up, and some kind of significant vibration occurred causing a further minor shift in either the terminal or the tails and BANG.

The bottom line is that a closing on a dead short or even a moderate load load holding an 80A fuse in your hand to complete the circuit is just damned dangerous, and the extent you are arguing against this basic fact leads me to believe you have no intention of ever taking it on board. I don't have anything further to add so won't be debating this any further.

With the greatest respect I won't be taking it onboard, I just dont see how you can guard against this, it was one of those "---- happens" moments and will more than likely never happen again in my lifetime.

 

[Mark42]

Blimey this thread has developed a life of its own! I only wanted some advice about a theatre installation

Arc flash? Pah! THIS is proper flash, my day job: video of me posted by a student last month. I had no idea that what I do routinely looks so stupid!

!!! I give up ... Can't get link or embed video to work !!!

 

[plugsandsparks]

More experience, I'm 52 and have had more than most.

Its no good asking this question, how can you test a cut out before you put the fuse back in, I'll pull plenty of fuses out over the next months and years an I'll put plenty back in again, I'm not about to do a full forensic on every little thing I do from now on on the off chance that a 0.00001% fault will occur. I'd rather take my chances.

.

 

[Mark42]

The fusing characteristics of the main fuse means a huge surge is needed for it to blow instantaneously, ...

Thanks for the info. I'd forgotten all that as I haven't messed with cartridge fuses for a long time, and had also forgotten where to look it all up!

I now think there's no need for me to worry, I was just checking in case I'd missed something. There won't be continuous overload, and any initial surges would be very short-lived.

 

[lozarus]

I still don't get all the hoo-ha about it though, it seems some people are ---- scarred of a big bang, you see it when someone is going to cut a cable which might be live with insulated snips, try as they might, they just can't bring themselves to do it even though they know they are perfectly safe from injury, its the bang you see.

You ever actually done it though? Colleague of mine inadvertantly did and it blew a hole in his CKs.

I used to be like you and hate on the hi-viz brigade. Now im one of them. Funnily enough we've never slowed a job down, sent workers home or distrupted anyones salary. They've all been completed to schedule - there's just been a lot less big bangs.

Not sure why you talk about guys having to pay mortgages. Is the job is slowed down due to H&S concerns (which literally almost never happens) the guys still get paid. Obviously.

Its all irrelevant anyway as this has wind up written all over it.

 

[Mark42]

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. ...

Yup, everything is LED. No halogen or discharge lamps at all. Filling the truss with Rank Strand 1000W luminaires off EBay really would be the way to blow the cutout off the wall.

The picture I had to hand is old. Here's the rig as it is today; I'm still adding stuff. This is all new to me so any advice from a theatre/rock gig chap would be appreciated!

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. ...

Good point. The loads are split across five circuits, each with its own RCBO.

There are three keyswitches for lighting and one each for audio and the projector, so parts of the rig can be rented to users separately, at different costs.

The keyswitches operate via cascaded 12V relays and mains contactors (that was all I had in my shed - this was a voluntary job for a charity and I donated many parts. That hurt!) No doubt I'll be told cascading those relays is bad practice. Which is probably right.

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

Ha Ha! Yes, probably. I LIKE mirror balls: they remind me of my youth . In heavy haze they look pretty cool actually.

I've ordered ten LED 2.5 degree sharpys which will be fun for the rock and EDM gigs we'll be doing ourselves. Where would a pro rig them?

There are a couple of other trusses at the other end of the the hall. Not sure what more to put on them yet. Any ideas?