3phase lighting and shared neutral

[industryspark]

Had some more thoughts on it today and flicked through my regs. If we look at the definition of a circuit

''An assembly of electrical equipment supplied from the same origin and protected against overcurrent by the same protective device(s)''

Note the (s) on the end. This could be our 3xSP mcb's could it not?, not indifferent from a Bs88 fuseboard where the fuses can be removed individually imo.

I think the installation meets the definition of circuit hence complies with the one shared neutral.

BS7671 points to this setup with the previously mentioned reg 559.6.2.3-the only contentious issue i can think of is the 'device' this reg speaks of. Is it reffering to OPD or will a simple isolator also be a device?-i suggest it will. It doesnt give an indication of where this device needs to be located in the circuit either.

The neutral conductor is sized the same as the line conductors and adequate to carry any expected current (including harmonic) regardless of whether 1,2 or 3 rows of lighting are switched on.

Thanks for the great response so far, an interesting topic.

Regards

 

[333rocky333]

not indifferent from a Bs88 fuseboard where the fuses can be removed individually imo.

Only ever seen that done for a 3 phase load with 1 neutral.

I would feel sorry for the engineer up a cherry picker, seeing the row off assuming it dead, testing phase to earth dead and then removing the socket and getting a belt off the N,or as tim says frying the units.

The use of 3 phase and N sockets at each light may overcome this and tap off each relevant phase for each light.
Anyone then working on the system will then be aware of what you have done.
Similar to using 3 phase and N track.
Where the N cant be safely parted without isolating all 3 phases first.

 

[holmslaw]

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[Spark123]

We extensively use BS88 boards. A 3 phase circuit does not always have 3 or 4 pole protection. We rely on drawings, schedules and labelling to identify where they are fed from.
Where the board is being used to supply single phase circuits the phase and neutral for each circuit is terminated in that board.

 

[bernardgreen]

So if someone decides to install lighting spread across all 3 phases with a common neutral it is a 3 phase circuit , and it requires 3 or 4 pole isolation/protection to all parts of the circuit.

And if someone decides to install lighting spread across all 3 phases with a individual neutral for each circuit , then it is just a group of individual single phase circuits, that require 1 or 2 pole isolation.

That is correct.

Anyone employed as a spark who cannot understand these basic facts should consider a career move to mcdonalds.

But not in the electrical installation office.

 

[ban-all-sheds]

I would feel sorry for the engineer up a cherry picker, seeing the row off assuming it dead, testing phase to earth dead and then removing the socket and getting a belt off the N,or as tim says frying the units.

Surely as Eric says the procedure for safe isolation would be documented and the permit to work would require it to be followed?

 

[plugwash]

Unfortunately in the real world there are so many risk assessments, permits to work etc that people just gloss over them and sign them without thinking too much. Or they just do the work without ever officially recording that they are doing it. That is why I would suggest putting a warning on the actual box where it is far more likely to get read than in some risk assessment/permit to work document.

 

[ban-all-sheds]

Unfortunately in the real world there are so many risk assessments, permits to work etc that people just gloss over them and sign them without thinking too much. Or they just do the work without ever officially recording that they are doing it.

Isn't that how people get killed, or make themselves liable for imprisonment and unlimited fines?

 

[ColJack]

We extensively use BS88 boards. A 3 phase circuit does not always have 3 or 4 pole protection.

but they should.. the use of bs88 fuses for protection does not preclude the necessity of local isolation where the 3 poles are linked.. that's what BILL switches are for...

 

[plugwash]

Unfortunately in the real world there are so many risk assessments, permits to work etc that people just gloss over them and sign them without thinking too much. Or they just do the work without ever officially recording that they are doing it.

Isn't that how people get killed,

Oh probablly, the point is that you really don't want someone getting killed on your premisis even if it is thier own stupid fault for not following procedures. So if you have dangers that people are unlikely to anticipate it's a bloody good idea to put warnings at the point of danger as well as burried in documents.

or make themselves liable for imprisonment and unlimited fines?

Usually the investigation only happens after someone gets seriously injured or killed.

 

[industryspark]

holmslaw wrote:

I find it depresing and hard to believe that some "qualified" electricians cannot understand the difference between a 3 ph and 1 ph circuit.

I dont know if that is aimed at me or anyone else who's posted but whats wrong with discussing the merits of an installation?

Anyone employed as a spark who cannot understand these basic facts should consider a career move to mcdonalds.

Come on holmslaw, maybe a good idea would be to climb down off your high horse and just stick to the topic instead of making things personal.


The problem with this installation is that its probably 20 odd years old and was put in with little thought. Now, because ive been tasked with replacing some of the fittings on the circuit i also got thinking about the merits of the circuit when i saw how it was wired. The fittings are supplied via a 3 core flex dropped down from adaptable boxes which have been used to break into the 3 phase & Neutral circuit. Because of this, disconnection of the neutral conductor in the way that it would be dangerous isnt really an issue in simply changing fittings.

Besides which, a proposed plug and trailing socket system between flex drops and fittings would make the removal and replacement of fittings safer still.

I would love to be able to rip the lot out and run complete new circuits but access into the roof isnt available for a long time in the future.

With this in mind, me being the only spark there am left with a decision, can i see if i can improve the circuits and maybe get them into compliance or do i leave a factory with only 50% of the lighting working when guys below are struggling to see when using heavy machinery?

Whichever way you choose you could argue safety is at stake. Whilst i agree that there are some issue's with the compliance of the circuit to the regs, i dont think they are strong enough for me to say ''im not touching it''

Ericmarks idea of labbelling and permit to work on the circuit seems like a sensible one. Any competent spark could look at the setup straight away and see how its wired thus being able to avoid the potential hazards-if they couldnt see that, then why are they changing fittings anyway?

559.6.2.3 (off the top of my head) calls for a TP device for isolation. This circuit in effect has that, although i agree the 3 single pole devices after are the 'problem' area if you like. So, changing them for a TP mcb would maybe help make the circuit compliant but would also leave the entire workshop without lighting in the event of a failure on one phase. Its lose/lose either way.

Regards

 

[ericmark]

The same questions have been asked on IET Forum with same mixture of replies. The motor controls of this

type often have built in protection for under voltage and phase loss they are made by a number of firms.
There are many other ways of increasing protection and safety. But I would say there needs to be a three or four pole protection device or relay but as I said before when approaching this type of problem risk assessment is the way to go so if anything does go wrong you can prove you took all reasonable steps to prevent danger.

 

[industryspark]

I will certainly look into those ericmark, thanks for your input. Its been an interesting topic though with mixed views. Thats the benefit of having the forums i guess.

Regards.

 

[Spark123]

We extensively use BS88 boards. A 3 phase circuit does not always have 3 or 4 pole protection.

but they should.. the use of bs88 fuses for protection does not preclude the necessity of local isolation where the 3 poles are linked.. that's what BILL switches are for...

A BS88 board cannot provide all pole isolation hence why labelling, drawings, schedules and the likes is a necessity. Where isolators are provided they are not always at the distribution board end either, they are often with the current using equipment.

 

[holmslaw]

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