Three phase safety

[John In Brum]

Hi All

 

I am trying to keep on top of technical issues in my theatre whilst having a background principally in sound.

 

Our lighting system is a fairly old three phase setup with wiring arranged to prevent the mixing of phases to an individual lighting bar.

 

With the recent acquisition of a number of LED fixtures they are being rigged and then powered from the nearest 13A socket. My feeling is that this is dangerous, and that really we should arrange for fixed 230V 13A sockets on each bar wired from the same phase as the dimmer channels and probably with RCCD's.

 

Your comments please on accepted practice.

 

 

 

 

John

[paulears]

While the concept makes sense, practically, nowadays with distribution of power very much done on the fly, it's pretty well impossible to keep sockets on a bar on the same phase. Indeed, the HSE's advice to musicians in a popular leaflet advised this - they're now reviewing it because 6 channel dimmer packs are now designed to be either single phase, or a phase per pair of channels. Some have multichannel Harting or Socapex outputs, making single phase per internally wired bar impossible.

 

The danger of finding yourself across multiple phases in a fault condition requires more than one simple fault, so the feeling nowadays is that since phase separation is not a requirement - the 6ft rule having long been retired, then it's a low priority in the safety stakes. The only reason I may organise power so a single mcb powers an entire bar, is simply because one switch can switch it off!

 

If you look at touring dimmer systems of the Avo type, with built in patch panels for power, it's clear that separating phases has no practical advantage any longer - if indeed, it ever had one.

[Guest]

I know the problem!- I've come across a similar issue in a local theatre near me.

 

It's a good idea to keep phases separate, but not vital. At the end of the day 3 phase flex has 3 different phases packed right in next to each other. As long as the bars are properly earthed, and your equipment is all properly maintained and PAT tested, running your 13 amp power from potentially a different phase is as I understand it fine.

I have come across a drama studio hard wired with different phases running down the same internally wired bars!

 

Edward

[alistermorton]

RCCDs are what you might call a good idea anyway, though :-)

[adam2]

Agree, mixing phases is pretty much a non issue these days, in order to receive a 400/415 volt shock no less than 4 faults would have to occur at the same time.

 

The regulation requiring 6 feet, later 2M between different phases of an AC supply or different poles of a DC supply was abolished many years ago. Even when it was a requirement, enforcement was patchy for fixed equipment and almost non existent for portable appliances.

 

Elsewhere in Europe, 3 phase is widely used d0m3sticly and no one worries about outlets on different phases being nearby.

 

 

 

[kerry davies]

In the beginning girder cranes used to pick up power from four bare copper wires mounted on insulators running the length of the workshops inside an I-beam recess. Innocent bystanders (Me!) used to get 440V belts when idiot crane drivers unthinkingly switched on power that had been turned off. Hence separation of phases wider than average arm span.

 

Things have changed, technology has moved on and what was once necessary is no longer the case. A good example is the explosives regs governing film stock which were, until recently, a hangover from early celluloid. I hope that helps with some background.

 

E2A why do you think I ended up as a safety man? I had or saw every accident known to man before HASAWA was even thought of. It was hairy back then.

[Jivemaster]

If you need to be worried about phase separation then go for a FULL internal system overhaul. Actually INSPECT every plug and socket, and actually check every lead or appliance. It takes reputedly FOUR coincident faults to cause a line to line fault in a system so eliminate all the faults and you are fine.

[ianknight]

 

Our lighting system is a fairly old three phase setup with wiring arranged to prevent the mixing of phases to an individual lighting bar.

 

With the recent acquisition of a number of LED fixtures they are being rigged and then powered from the nearest 13A socket. My feeling is that this is dangerous, and that really we should arrange for fixed 230V 13A sockets on each bar wired from the same phase as the dimmer channels and probably with RCCD's.

 

Your comments please on accepted practice.

 

 

The only problem is that by having 13a sockets added to your existing lighting bars, the installing electrician may feel that the existing wiring isn't up to scratch and insists on removing it all or replacing it!

 

Yes - the ideal situation is as you describe (and is certainly as we install LED fixtures in rigs) but unless you're ready for a large chunk of money to flow away on wiring and containment without warning then for now - use the 'floating' extension cables and perhaps use RCD adaptor plugs. I would however buy one to try first - depending on the internal circuit design of the plug, they may not react well to the switch mode supply in the LED's - generally we only fit MCB's and leave the 13a plugs fitted to the LED's (so the plug and its fuse protect the cable to the fitting).

 

Until you get to a final re-arrangement of your rig that makes everyone feel safer, there are measures you can use to mitigate the possible risks (as they stand)...

1. 
    
   
2. Don't have the LED's powered up whilst you focus the conventional lanterns
   
3. Use a flash lead (an extension cable fitted with a socket to suit your conventional lanterns) to focus lanterns and don't power the dimmers
   
4. Ensure your PAT testing is up to date - this will "catch" the great majority of faults that are likely to cause harm
   
5. Be ruthless when selecting cables and lanterns to rig and don't accept "Oh - that's only a little cut in the cable..." even from your own internal voice B-)
   

 

Ideally, we'd love you to rewire your lighting system but knowing your theatre and the financial constraints you operate under like a lot of "little theatres" - these things are costly, time consuming and need to be planned/budgeted for.

[Jivemaster]

I don't see any problems if all the test and inspection is done, and the inspection will cover most possible faults.

 

There is NO need to get a company in at 25p an item to stick stickers on the easy bits and miss the rest, there is a LOT of need for you and your helpers to find everything and look it over for condition and connectors and fuses as was intended.

 

It's actually quite normal for there to be several connector styles. One typical one is 15a for dimmed power and 16a for hard power, it seems that you want to use 15a for dimmed power and 13a for hard power, -if it all passes visual and testing- what's the problem. You'd use 15a 16a, Soca and Harting if you needed!

[adam2]

Provided that the existing dimmed circuits are correctly wired and in good condition, then adding some additional 13 amp or other sockets should not require any alteration of the existing.

 

Most LED fixtures are of low loading and significant numbers may be worked from existing 13 amp sockets. Simply make certain that the extension leads are of good quality and undamaged. I recommend 3 amp fuses in plugs for LED lights, there is usually nothing dangerous in a larger fuse, after all many such lights are used on unfused 16 amp plugs and no one worries about that ! The merit of small fuses is that a defective fixture will hopefully only blow the fuse in its own plug and allow continued use of the rest of the rig.

 

 

 

[stories]

Hi All

 

I am trying to keep on top of technical issues in my theatre whilst having a background principally in sound.

 

Our lighting system is a fairly old three phase setup with wiring arranged to prevent the mixing of phases to an individual lighting bar.

 

With the recent acquisition of a number of LED fixtures they are being rigged and then powered from the nearest 13A socket. My feeling is that this is dangerous, and that really we should arrange for fixed 230V 13A sockets on each bar wired from the same phase as the dimmer channels and probably with RCCD's.

 

Your comments please on accepted practice.

 

John

 

As already said, lot's of dimming systems have 6 channels across 3 phases so you potentially end up with all 3 phases present on the end of a soca spider etc... so common use these days, however the supply is usually via an RCD which will offer some protection from cross phase faults.

 

My take is that there is no excuse in this day and age to not have your supply's protected with RCD/RCBO's any electrical installation, while not 100% removing any electrical shock/cross phase shock hazard, they significantly reduce the likely hood of injury. Surly the H&S at work reg's etc would mean employers should be actively reducing such risks when identified??

 

I would also, personally, be less concerned with getting a 415v belt (specifically) over getting any belt (240v only requiring a single fault) when working at height and the likely ensuing fall, which brings me back to expecting the supply(s) to have an RCD.

 

-s

[DrV]

...however the supply is usually via an RCD which will offer some protection from cross phase faults.

 

No it won't. A 3 phase RCD monitors the balance between the current flowing in all 4 wires. It will only detect an imbalance if some of that current bypasses the current transformer (by flowing to earth). So if you put a load (be it a real, intended load or an unfortunate victim's body) between phases it will not trip.

 

Dave

[smalljoshua]

If however each final circuit is protected via an RCBO, there would be an imbalance between L and N on each supply compared to that supply's neutral so should surely trip then.

 

Josh

[DrV]

If however each final circuit is protected via an RCBO, there would be an imbalance between L and N on each supply compared to that supply's neutral so should surely trip then.

 

Josh

True of course but I was referring to the mention of AN RCD (singular) rather than RCDs (plural).

Being a bit of a pedant perhaps!

Dave

[stories]

...however the supply is usually via an RCD which will offer some protection from cross phase faults.

No it won't. A 3 phase RCD monitors the balance between the current flowing in all 4 wires. It will only detect an imbalance if some of that current bypasses the current transformer (by flowing to earth). So if you put a load (be it a real, intended load or an unfortunate victim's body) between phases it will not trip.

 

Dave

 

 

Which is partly why I said 'some' not 'complete'. I would think a fault condition where the only path for current to flow is hot to hot leg is fairly slim??...

 

Your also assuming a 3phase RCD, rather than an RCD per leg. It's down to a qualified spark doing the work to decide what is best to achieve the desired protection vs cost, not an internet forum.

 

True of course but I was referring to the mention of AN RCD (singular) rather than RCDs (plural).

Being a bit of a pedant perhaps!

Dave

 

A little, yes, been an internet discussion, not a design document..

 

Though valid and useful to point out for other people reading, as RCD's don't 100% guarantee safety.

 

-s