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Topic Title: Making sure a circuit is not switched off
Topic Summary: How?
Created On: 01 November 2017 10:58 PM
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 01 November 2017 10:58 PM
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Zs

Posts: 3827
Joined: 20 July 2006

hello,

This is covert so I have to whisper. Get close to the screen.

This enquiry came to me this morning from the man who will soon be my boss. He is a top bloke and you will like him so very much. But it is all a bit cloak and dagger and there's a story involving every 'ism' you choose to mention apart from gender. Just shocking. But I have professional admirers in this place and my resignation was laughed at. When it moves into history and out of real time I'll share. Tough but give me a few weeks eh?

But for the time being, Top Bloke above is overworked and needs a Girl Friday. He sent me an enquiry on the home email and I'd like to be able to help.

I've not seen it but there is a circuit which must not be switched off.

It is protected by a breaker at the DB. the DB serves lots of other circuits.

I'm thinking Castell
I'm thinking labels that say 'Do Not Switch Off' (if not a touch naïve)
I'm thinking 60898 or 60947 in a locked container
I'm thinking of isolation switches by Craig and Derricott or even by Timeguard which have a slot for a padlock.

I'm not thinking of a padlock on the breaker but it was in the enquiry.

I'm also thinking of faults on the circuit and how they affect the DNO fuse.

I expect that this is lights which indicate that certain circuits are operational.

Do you mind helping me to save my career in this bonkers place by giving me your views on how to prevent a circuit from being isolated?

Zs
 01 November 2017 11:07 PM
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mapj1

Posts: 9559
Joined: 22 July 2004

instead of the MCB, you could maybe have a DIN rail fuse holder for the last man standing circuit, as it still provides some ADS, but has no flickable switch.


Perhaps also add some kind of power fail alarm. Years ago a friend and I modified an emergency light, so that instead of the light coming on a klaxon sounded. And have it fed straight off the bus, not a DB sub circuit - ideally in its private "always on" DIN box beside the DB, but without a main switch. At a push it can still be fed from the DB, looped to the wrong side of the big DB input switch.. Is a UPS a possibility - it is rather more conventional and guards against external power failure too.
edited to clarify.

-------------------------
regards Mike


Edited: 02 November 2017 at 08:54 AM by mapj1
 02 November 2017 07:06 AM
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AJJewsbury

Posts: 15997
Joined: 13 August 2003

I'm not thinking of a padlock on the breaker but it was in the enquiry.

Why not? Any modern MCB will be 'trip free' - i.e. it's free to trip even if the toggle is held in the 'on' position.

It is protected by a breaker at the DB. the DB serves lots of other circuits.

That sounds like a seriously weak point, even if discrimination downstream is 100% (which it sometimes isn't). If you really want the least unreliable approach for a mains supply I'd be thinking more along the fire alarm supply - come of the supply side of the main switch, separate switchgear, painted a bright unusual colour and labelled with dire warnings.

But in most places the DNO supply itself isn't 100% and no amount of careful distribution will cure that, so as Mike suggested a UPS of some sort might make a lot of sense. Depends on the load of course - if it's a hoist to lift a battleship you might be a bit beyond what a few batteries and an inverter could manage, but then there's local backup generation. A local UPS approach might also make sense if you're not wanting to draw a lot of attention to the load you're trying to protect.

- Andy.
 02 November 2017 07:42 AM
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aligarjon

Posts: 3867
Joined: 09 September 2005

Locking it away also makes it difficult to turn back on if the right people aren't around that know the score on certain days.


Gary

-------------------------
Specialised Subject. The Bleedin Obvious. John Cleese
 02 November 2017 08:57 AM
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mapj1

Posts: 9559
Joined: 22 July 2004

most of the fuseholder designs can be wired shut. (you could even have a DNO style henley, they make ones with smal fuses for lamp posts etc... )Cutting a wire seal is sometimes better than a padlock as anyone can open the circuit in a suitably dire emergency, but it is then a very deliberate act.

-------------------------
regards Mike
 02 November 2017 04:55 PM
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broadgage

Posts: 2433
Joined: 07 August 2007

Some protection could be obtained by taking two mains supplies from well separated parts of the installation and an auto changeover unit near the critical load.
That would protect against accidental or malicious turning off and also against any faults deliberately introduced with the intention of operating upstream OCPDs.

No such scheme can protect against the DNO cut out fuse operating or being deliberately removed, or being deliberately blown by a fault introduced for this purpose.

To protect against such eventualities requires a generator, or UPS, or a feed from an independent DNO service in another building.
 02 November 2017 08:44 PM
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AncientMariner

Posts: 717
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When I first read your post, I was going to suggest that whatever it is, would be better run on gas...

I think that it would be easy to over design here, the problem being then that the extra complexity makes a power failure more likely.

They seem to want 100.0% reliability, or do they? They ask for a circuit which cannot be switched off; a simple answer would be to not have any switches. Job done. But is that what they want?

I think that you need to consider what would happen if the circuit is switched off. There is a story that during the cold war, submarine commanders were told to assume that the UK had been annihilated if: a) they could not receive Admiralty radio transmissions and b) they could not receive Decca navigator radio signals (a now discontinued radio navigation system which used Low Frequency transmissions) and c) BBC Radio 4 on 198 kHz Longwave.

Does your circuit come into this category?

I can imagine a refrigerator which has to keep biological items below a certain temperature; or else.

When I was at sea, I was on a ship which among many, carried an integral refrigerated container which contained some nasty chemical which would spontaneously ignite if the temperature got over a certain figure. The container was powered from the ship's 440 volt 60 Hz supply system, which also powered all the other integral containers. However this container also had a back up diesel generator built in, so that on 440v failure the diesel would start up and a siren to alert personnel if it failed. This container was also stowed in a position where if push came to shove it could be jettisoned off the ship. Since for some reason refrigerated cargo came under my responsibility, I was really glad when this container was lifted off the ship, with diesel running in, if I remember correctly, Shenzhen, China. My relief was short lived since I was then contacted and asked if I would go to the container terminal to check the container since its siren was going off! Luckily all it needed was the alarm acknowledgement button to be pressed and all was well.

Some integral containers had two separate refrigeration systems, mainly to allow for compressor failure. (Some of the frozen cargoes were not what you would want to thaw out in the tropics. One which still haunts me, is 20 tonnes of frozen placentas for the cosmetics industry.)

My best suggestion is to keep it simple, Mike's suggestion of using suitable fuses rather than switchable mcb's makes sense.

If the critical equipment can withstand a short outage, perhaps two supplies as independent as you can get, and a changeover system - but keep it simple. Many years ago I visited the ITV Moel-y-Parc TV transmitter, this in VHF 405 line days. They had two 11 kV feeds, each from different 33/11kV substations, to the site on the top of their hill in North Wales.

Yes, perhaps easier if they can get a gas one...

Clive

-------------------------
Clive S Carver GCGI IEng MIET MITP
 02 November 2017 09:15 PM
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Zs

Posts: 3827
Joined: 20 July 2006

When I looked in here this morning there wa a post that said the internal cut out would work even if the switch was padlocked on the on position. I don't see it now so perhaps it's been removed but is that true?

Well Ancient M, this may well be fridges but I don't know for sure. They do have fridges containing some special film there, and I know that they are on a normal DB and are also in a basement so rarely visited. that may be it. It is a Schneider16A type C 60898.

I've had an email exchange today. Access to any keys is always through a rigorous procedure and these would be no different. But key location/ number is always marked. That even applies to DBs in there - always locked- and they sure wouldn't let my fabulous bunch collected by BOD over the years in

It seems the concern is that it could be accidentally knocked off and left off. I think they need to separate it into a separate DB and put the mother of all labels on it. Like you say, this could be over-engineered.

A key switch...just like for the cleaners sockets in blocks of flats might do the job too.

Mike, they have several of those fuse container types. I pointed them out too. Thanks.

I'm on annual leave at the moment doing some real work in Mum's Church. No doubt this issue will result in five hours of meetings resulting in 20 minutes work and job done?

JP sent me a picture from one of his jobs once, of a self resetting circuit breaker. You'd love it. The guy had screwed a spring to an MCB. One end into the top of the breaker, the other end into the toggle switch. You couldn't make it up. Can we post it on here JP? If so I'll send it to Mike who knows how to do these things.

Thank you.

Zs
 02 November 2017 09:31 PM
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Fm

Posts: 1733
Joined: 24 August 2011

These cryptic posts are getting tiring. What's the equipment, what's the environment.
 02 November 2017 09:44 PM
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leckie

Posts: 4390
Joined: 21 November 2008

The post about the MCB. tripping when locked on was by AJ, just below Mikes picture of the fuse carrier.

And an mcb will trip when locked on. If you push an mcb to on when there is a sufficient fault present, it will trip. Even though your hand is holding the toggle in the closed position. I am not advocating trying this out as a test procedure though
 03 November 2017 08:36 AM
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broadgage

Posts: 2433
Joined: 07 August 2007

If this is for fridges , i.e. more than a single fridge, then the use of a single 16 amp circuit worries me a bit.
I perceive a small but real risk of a fault in one fridge tripping the 16 amp MCB and thereby interrupting the supply to other fridges.
I would prefer a 32 amp circuit, so as to increase the chance of discrimination between the plug fuse and the circuit protection, or alternatively a circuit for each fridge.

Also what happens in a power failure, presumably the internal temperature of the fridges increases, and they all re-start at the same time when power is restored.
Has anyone assessed the chances of the starting current tripping the 16 amp MCB.
 03 November 2017 08:46 AM
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AJJewsbury

Posts: 15997
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When I looked in here this morning there wa a post that said the internal cut out would work even if the switch was padlocked on the on position. I don't see it now so perhaps it's been removed but is that true?

If that's my post, it's still there. Do verify for yourself, but the definition of 'trip-free' and the requirements of section 8.1.2 both in BS EN 60898 might be one place to look. (or manufacturer's data if you want clearer wording)
- Andy.
 03 November 2017 08:48 AM
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dgmeica

Posts: 42
Joined: 08 October 2006

Small UPS at the load end? Doesn't matter what happens upstream be it maintenance, malicious or DNO issues....
 03 November 2017 02:06 PM
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Zs

Posts: 3827
Joined: 20 July 2006

Thank you Andy, I didn't know that and it is most useful.

I'm not back there until next week so I genuinely don't know what the equipment is. Fm, I sent you a link. I have been put on admin duties 8-4 amidst the 'Life on Mars' series type of politics so these enquiries about interesting things happen over cups of tea and email for the time being. I'll bore you with the story when it is over.

The cup of tea meeting about this is already booked for Monday though so I'll find out more.

Zs
 03 November 2017 06:22 PM
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AncientMariner

Posts: 717
Joined: 14 December 2004

If it is a fridge, check that there are no unexploded bombs in the vegetable draw. http://www.bbc.co.uk/news/world-europe-41857694

Very cunning these English military types...

Have a good weekend.

Clive

-------------------------
Clive S Carver GCGI IEng MIET MITP
 04 November 2017 07:44 AM
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davezawadi

Posts: 3896
Joined: 26 June 2002

I think a fuse direct from the meter tails is the best way, if you want an isolator then a padlocked one is the way to go. Any switching such as sub mains, or other DBs is obviously out, as someone could turn them off without knowing that the circuit must be kept live. A suitable notice should keep it under control, with a contact number for the padlock key, so that a responsible person knows what is happening. All this changeover stuff will reduce reliability, and if the site power is lost then surely there will be a backup generator? I discern that this site probably has many safety critical sections so the necessary infrastructure is surely in place. This is a case of designing a solution, so forget the exact words in the BYB, just sign on the dotted line.... Remember too that other faults could take out anything other than a DNO fuse, they very rarely fail, so that good design of the rest of the installation is also important. You could fit a neon indicator to show that the circuit is live at its fuse, so that checking that all is well is easy, with a label saying something like : Circuit live when indicator is lit. You won't fail with that lot!

-------------------------
David
BSc CEng MIET
david@ZawadiSoundAndLighting.co.uk
 04 November 2017 07:53 AM
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aligarjon

Posts: 3867
Joined: 09 September 2005

Some fridges have an alarm at power loss, be it a visual flashing light or a sounder. With todays tecnology ( cant spell) i would think an alert to mobile could be set up etc.

Gary

-------------------------
Specialised Subject. The Bleedin Obvious. John Cleese
 04 November 2017 08:24 PM
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KFH

Posts: 554
Joined: 06 November 2010

A customer of mine has a nifty device which send him a text if the power goes off in his garage which is some way form the house and has his freezer in it.
 05 November 2017 10:41 AM
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AJJewsbury

Posts: 15997
Joined: 13 August 2003

A customer of mine has a nifty device which send him a text if the power goes off in his garage which is some way form the house and has his freezer in it.

I know of one or two computer rooms with similar systems (monitoring all sorts of system parameters) - the trick is with these systems is to make sure that the account behind the SIM card it uses always has enough credit... seemingly is it's easy to forget about and then the messages don't get through just when they're most needed.
- Andy.
 06 November 2017 09:09 PM
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Zs

Posts: 3827
Joined: 20 July 2006

Evening,

I'm struggling to find back-up evidence of trip free MCBs on low voltage. Not disputing, just can't find it in writing.

I can find a really good article from Eaton for 600v plus and another for underground circuits. But as yet nothing on a straightforward low voltage breaker, for example a 32 A type B or C.

Do you know of a reference to which I may refer please?

thank you,

Zs
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