Anyone for a widowmaker ?

[SimonH2]

Somewhere in a box in the loft I have a BS 1363 plug with a length of flex from it - on the other end of the flex are a couple of spade connectors.

And ?

And it's not made anybody a widow yet.

Or widower.

Let me be sure I understand you right.

You seem to be suggesting, despite all your lecturing to others about the importance of never ever deviating from laid down standards and safe working practices, that you consider a lead with exposed live terminals/ends to be OK for the sort of situation under discussion because your lead hasn't caused anyone any harm

 

[SimonH2]

Another possibility to consider, what if you locked off the main isolator and then just connected the cable from the generator to a spare way in the DB?

More or less exactly the same issues arise. It is something I have done myself - and I was very careful to make sure none of the below happened (the genny was physically right next to the board so I could see that nothing untoward was happening).

You could still close the main switch onto a mains supply while the alternate supply cable is connected - thus back feeding the genny or making the exposed live pins of a free plug live.

You could still close the main switch onto a "dead" supply while the genny is attached and running, thus back feeding the network - this is exactly the sort of thing Westie101 and his colleagues are concerned about.

You still have the stage where you want to disconnect the genny supply and are then left with live, bare tails. Unless you take suitable precautions, this is itself a risk - someone could plug it back in and power it, they could plug it in elsewhere, ...


So once again, yes it can be done safely (as safely as anything can be when it relies on a fallible human element in the safety case). But there are safer ways of doing it (ie a changeover switch and correct gender of inlet - inherently safe by having an effective interlock), and it sends a false message to non-technical onlookers that it's acceptable - hence increasing the risk that they'll do it themselves without knowing the steps required to make it safe.

 

[westie101]

It's a subject we often discuss, if the work is on the LV underground network we normally work as though it were live even if proven dead.

The HV underground network will always be isolated from the LV network and connected to earth

It is the overhead networks that can be the biggest problem, however we now short the LV conductors together and to N/E so any generator connected could have a short but happy life before it trips, similarly on the HV overhead we either isolate the LV or short the HV conductors together and connect to earth between any possible source of supply and the point of work.

All in all it pretty much ensures our safety, but we would still be unhappy with any unusual connection.

 

[ban-all-sheds]

You seem to be suggesting, despite all your lecturing to others about the importance of never ever deviating from laid down standards and safe working practices, that you consider a lead with exposed live terminals/ends to be OK for the sort of situation under discussion because your lead hasn't caused anyone any harm

No.

 

[SimonH2]

It is the overhead networks that can be the biggest problem, however we now short the LV conductors together and to N/E so any generator connected could have a short but happy life before it trips, ...

The worst case I can think of is if there's been a cable fault fairly close to a small group of subscribers, and you need to splice in a new bit. If it's bare overhead wires then you can get at them to apply shorting/earthing links, but not insulated underground cable without breaking into the insulation. If the cable to a small number of subscribers has broken, then it wouldn't bee too hard for one user with a genny to liven up one phase ready for the cable jointer to come along ... If it's a farmer with a bigger genny, then liven up all three phases.

At my last employer, we used to have the power off every few years for trees to be cut back along the overhead line. We had our own 11kV/415V 500kVA transformer on site, and they used to send someone along to open the breaker and make sure we couldn't back feed - a genny big enough to run our site would have had no problem backfeeding and supplying a handful of houses and farms further up the cable.
It was quite interesting chatting with the guy that came last time. Got to peruse the local network diagrams which explained all our power cuts - we were on the very end of a route that seemed to take a tortuous route via every small village in the area before it got to us (all overhead).

But digressing, he told me a rather amusing tale about some works on a supply to a local town. Rather than cut the town off for a day, they brought in a couple of big gennies, connected them up, fired them up and synced them to the grid - and then they sat for an hour or two, running but not under load.
This chap says he saw his boss, covered head to toe in black oily crap. He just happened to be walking past the gennies when the supply switch was opened and the gennies took up the load. Running without load for a while, the engines had (like most diesels) got cool and oily in the cylinders - and suddenly the taps were opened and all this black oily sh*t went out the exhaust, where the manager was just passing

 

[Lectrician]

I am pretty sure our DNO do not have gensets capable of syncing these days - there will always be a blip in the power while they transfer to a gen.

In saying this, the network is built for redundancy, and the use of generators is very low - If there is some switchgear or cables to be worked on, the loads can be trasferred elsewhere, all the additional routes being 'in phase' with each other, so they can be turned on and then the section to be worked on turned off.

You usually have 11kv rings, and often these rings have two sources from the 33kv network. There are often interconnections between the LV, 11kv, 33kv and 132kv networks allowing you to switch and move loads all over the place. You often see on the LV network fusecariers up the poles with the fuses out and cable tied to the carrier. You know that this is one of the interconnections.

Widow Makers - Reminded me of this from a couple years ago, spotted on site.

 

[Lectrician]

Try and work this one out - A changeover attempt on a small farm.

We did some work on a site recently which had a 100kva TP generator and an automatic transfer switch. It was a very old transfer switch and was actually only 3 pole, not 4.

Thing is, the mains supply was TN-S, but the generator (as usual) was TN-C-S. This means you have a N - E link on the TN-S supply, and as such current will ALWAYS flow between this fault when you are drawing current. Site maintenance advised of the issue with this, and replied with "ah, thats why it sparked like hell when we moved the generator the other year"!

 

[SimonH2]

I am pretty sure our DNO do not have gensets capable of syncing these days - there will always be a blip in the power while they transfer to a gen.

Well that's also possible, either way, the gennies were reported as idle for a while, then the taps were opened.

In saying this, the network is built for redundancy, and the use of generators is very low - If there is some switchgear or cables to be worked on, the loads can be trasferred elsewhere, all the additional routes being 'in phase' with each other, so they can be turned on and then the section to be worked on turned off.

You usually have 11kv rings, and often these rings have two sources from the 33kv network.

That will vary with the area, and I guess the age of it's infrastructure.

I know that at home we have 11kV rings as described (dual 132kV feeds, dual 132/33kV transformers and switchboards, dual 33/11kv transformers and switchboards, 11kV rings will an end on each side of the board, and a link to allow both sides of the board to feed from one transformer/supply) - so yes, lots of redundancy at the local level. I've had a tour of the local substation while my brother was doing some commissioning work on new 11kV switchgear.

On the other hand, that doesn't completely prevent long power cuts. It was a long time ago, but we had one at home which lasted a whole day due to a 132kV fault. Unfortunately, the other feed was isolated and earthed for maintenance work ... Took out the whole area except for the town where I work - that has an extra feed because of the nuclear shipyard.

At my previous employer - they are very much at the end of a long chain with little by way of redundancy.
As an aside on that, they were offered (for a price of course) the facility to be switched over to the the village supply - when our power was off, you could usually see all the lights still on in the village. I pointed out that should a fault occur, we'd need to wait for the engineer to come out and switch us over ... and what would the on-call engineer be doing if there's a fault ? Well I guess he'd be out dealing with the fault, and would be available to switch us over after he's dealt with that.
Management kept asking about a genny every time there was a power cut, but would decide it was too expensive ... until the next power cut when they'd ask again

 

[westie101]

We did the support a town with 2 x 1650 sysncronised sets some years ago.
Day 1 set them up
Day 2 ran them and synchronised to the grid to test them (brand new sets so there were teething troubles) then shut them down
Day 3 started them up, well actually only one of them as the other made some very strange noises from the engine. Shut them down and the faulty one taken away for repair!
It was a CAT V16 engine in 4 banks of 4, overnight one bank had filled with fuel so on starting the head had blown off that bank!!

On their return we tried again (they got here even though one of the trailers lost a pair of wheels on the A66 - that is lost, they came off and were never seen again!!)
Started them up and the newly repaired set was like a steam train as it burned oil and fuel off in the exhaust pipes, 4 of them vertically mounted
Now this is where your story possibly came from, the CAT engineer with the sets removed the drain plugs on the botton of the vertical exhausts and, yes, covered everything within 20 metres with black oil spots including people, vans, cars ....
(they were hired through a generator company based in that town that builds nuclear subs)

The ability to sync sets on is dependant upon their size, we had some 100kVA units that could sync, but were never used in anger to do so.
At present in Cumbria there are 11 100kVA non-sync sets (there were 12, but that is another story) and 3 200kVA syncable sets. Stationed at Blackburn there are 4 500kVA syncable sets.
We also have a major hire company on call that can usually deliver in 4 hours anything up to 1650kVA

 

[Spark123]

I needed spades on mine - I had a UPS with the utterly brilliant design feature of needing the batteries to have (some) life in them for a relay to operate the charging circuitry. Batteries which were (too) dead could not be charged without a bit of bypassing. There probably was a good reason, but it was a tad annoying when trying to resurrect the thing.

We have some of these but we just take the top off the relay and poke it with a stick

 

[plugwash]

The worst case I can think of is if there's been a cable fault fairly close to a small group of subscribers, and you need to splice in a new bit. If it's bare overhead wires then you can get at them to apply shorting/earthing links, but not insulated underground cable without breaking into the insulation.

But the DNOs have procedures for cutting and join those cables while live so "work like it's live even though you think it's dead" should solve that problem.

 

[SimonH2]

OK, an update ... prepare for "slap forehead with palms" moments ...

Got a phone call from customer when the power went off. They can't get the site running off the neighbour's genny - as soon as they connect, "it trips". Don't know more than that rather non-techincal description. In any case, it's not our problem and they've already called the spark back.
Cue remote shutting down of their servers before the UPS batteries expire, and me popping home to collect genny (again) - no chance of getting there before batteries run out. As it happens, by the time I get there, the power has been on for a few minutes - so all I have to do is press a few buttons to start stuff up again and hang around in case anything fails to come up properly (I did have some other stuff to do on-site).

Anyway, I had a wander across to see whats-what - mostly to see where the two cables from the socket went to, answer being into one plug at the genny end. Guess there weren't enough cores in one cable - remember that snippet.
Next door (the ones with the genny) have another trailing lead off the genny, with a separate single core trailing alongside it - guess they were a core short. And the end of the cable has, ta-da, a plug plugged into a red socket
One of their guys was there and we got chatting, though he didn't seem that technical. His side of the story went something like :
"We've never had any problem before, we just open the main switch and fire this up. It's only when we connected next door. Our machines don't use a neutral so it's not connected in the cables. All the breakers in the board are 3 pole, so we've never bothered, we pick the neutral up from the substation. We've added the neutrals to the cables, and dropped the neutral out of our board and it's all running now."

Now, these are quite modern industrial units, so I can't see them having dodgy electrics where the main switch doesn't isolate the neutral. So they've been running their office with a floating neutral, guess they've just been lucky ! But I can't see why that would make the genny trip when next door was connected (also without a neutral at first).
Anyone want to speculate what was going on - apart from the widowmaker and 4 core power leads with 5 pin plugs and sockets ? As far as I could tell from what people said, it was something at the genny end tripping, but don't know what.

 

[SimonH2]

Oops ...

Now, these are quite modern industrial units, so I can't see them having dodgy electrics where the main switch doesn't isolate the neutral.

... as I remember that 3P boards don't always have a switch pole in the neutral.

So they'll have been getting a neutral via the earth wire in the flex, a neutral-earth link in the genny (I assume), and the neutral-earth link in the TN-C-S supply.

Supplementary question, just out of interest ...
Is it more common to have a switched neutral or more normal not to have ?
If 3P boards don't always switch the neutral, why is this different from 1P boards where I can't recall seeing one without a switched neutral. The only reason that comes to mind is that on a 3P board you may not notice a faulty neutral if you have a well enough balanced load - though that seems a fairly marginal reason.

 

[westie101]

A lot of generators have a 100mA RCD built in so that if strange things are done with neutrals/earths they will quickly trip.
Any we use have this facility isolated as we often connect the neutral & the earth to the same point.

Certainly in the case of a generator the customer should provide their own earthing and neutral earthing and not use ours, as you can imagine if we are working on the local substation and start disconnecting cables etc. the risk of losing the neutral earth are high.

I'm sure that for a unit of that size the customer should have installed their own earthing system and must isolate from ours when the generator is operating. I'll check next week.

 

[RF Lighting]

Supplementary question, just out of interest ...
Is it more common to have a switched neutral or more normal not to have ?
If 3P boards don't always switch the neutral, why is this different from 1P boards where I can't recall seeing one without a switched neutral. The only reason that comes to mind is that on a 3P board you may not notice a faulty neutral if you have a well enough balanced load - though that seems a fairly marginal reason.

I'd say that 90% of the TP&N installations I've worked on / installed, do NOT have an isolator pole for the neutral.

Some larger switchfuses and panel boards have a neutral link you can disconnect for testing / maintenence purposes, but not for general isolation.

I'm not really sure why there is a difference between SP and TP installations TBH. Maybe someone else knows?