Bare 50Hz AC 3 Phase Bus bars...what is safe voltage to use

[treezleef]

Hi Hi,
We will have isolated three phase bus bars. Fully Mains transformer isolated. (mains input is 240VAC phase to neutral)
We're going to step down the 3 phase mains to the bus bars using a 50Hz 3 phase transformer.
What is the highest VAC level that these bare bus bars can be at and not be a danger to humans touching them?
(ie touching one bus bar, and also touching between two bus bars with each hand)

Also, i presume we have to have the neutral output of the three phase transformer earthed...otherwise the 3 phase output will be floating and could potentially float up to a dangerous level?

 

[Darkwood]

I use exposed voltage parts on machines sometimes to ensure production stopped if the conductive part of the machine breaks and touches down... Basically I have a wire drawing machine and we fit a monitored copper ring to pass the wire through, if the wire breaks it touches the ring and shuts machine down.

I actually use a 2 stage isolation method here, I use a monitored 24v dc supply that powers a float level unit, that unit dishes our about 6v DC and I have thus a safe voltage at extra low voltage that even in component failure will not rise to any dangerous level.

I would not be floating any voltages for public interaction in excess of voltages you find in a typical household battery, also you need to ensure it is fail safe IE a failure of the component or device reducing the voltage could not inadvertently raise the potential to dangerous levels.

If you expand a little on what you are exactly trying to achieve here there maybe a better solution as in optical sensors etc that can safely mimic or achieve the same outcome needed, a little more expensive but can remove the need to energise anything and thus you can even use insulated materials and not expensive copper bars.

 

[treezleef]

Thanks, yes, its just to light up the multiple exhibits. 10kW worth of 40W LED lamps is required.
The bare aluminium bas bars will be overhead, so people can touch them but only if they reach up.
We will have signs up saying "don't touch the bus bars" and i assume that will get us through the regulations?
The bus bars are bare metal...not covered.
So up to 50VAC is OK?

Low voltage DC just means too much current for us.
The current in the bus bars cannot be limited...as it has to supply to all the 10kW of lamps.

 

[westward10]

You cannot use out of reach as a measure of protection notices or not.

 

[DPG]

You need to be careful with this one. 50V at high current will give a big spark.

Who is installing the switchgear?

 

[treezleef]

Thanks...switchgear?.....you mean the switch to the primarys of the 10kW three phase transformer?
Our electrical guy i presume is doing this.
I presume it will be a big contactor with a biggish manual lever and these arent that expensive i hope?

The Art Gallery contains high price exhibits and the general public are not allowed in......only film crews who are under strict supervision. We have staff working in the gallery...and they will be trained not to touch the bus bars so i presume since its a "controlled area" then we will be allowed to have bare 50VAC bus bars as described?

We have a prototype, and we can touch one of the bare bus bars (with isolated 40VAC in it at the moment) when its energised and it does us no harm (mind you we have rubber souled trainers on). Mind you, i've never tried grabbing one of the three phase bus bars with one hand, and another phase with the other hand.
What would this do, i dont want to find out.
Mind you, i have taken non isolated 240VAC from hand to hand before and it didnt kill me...but i could feel my pectoral muscles vibrating!...no pain though....i was trying to press some probes across the 240VAC, and the contacts were corroded, so i had to apply loads of pressure, and then my hands slipped...and zappppppp!

But i guess i want to know which regs apply to us.?..and how can you get a "clause" in those regs so you can do what we want to do (ie 50Hz AC in three bare bus bars)?

 

[Lucien Nunes]

3-phase delta, 50V line-line, will need 115A to supply 10kW. At those currents, small metal objects that might get across the busbars, a bracelet or watch strap, the frame of a pair of shades that someone acidentally launches upwards, etc, could be heated to red heat almost instantly and/or rain down a shower of molten metal droplets.

Telephone exchanges, where only instructed technical people were allowed in, made widespread use of 50V busbars. Anything that could be touched or shorted was insulated.

This does not sound like a realistic project to me, and I suspect the gallery's insurers would agree, if they exist!

 

[ipf]

What was the original purpose for the bus bars?

 

[rapparee]

Thanks...switchgear?.....you mean the switch to the primarys of the 10kW three phase transformer?
Our electrical guy i presume is doing this.
I presume it will be a big contactor with a biggish manual lever and these arent that expensive i hope?

The Art Gallery contains high price exhibits and the general public are not allowed in......only film crews who are under strict supervision. We have staff working in the gallery...and they will be trained not to touch the bus bars so i presume since its a "controlled area" then we will be allowed to have bare 50VAC bus bars as described?

We have a prototype, and we can touch one of the bare bus bars (with isolated 40VAC in it at the moment) when its energised and it does us no harm (mind you we have rubber souled trainers on). Mind you, i've never tried grabbing one of the three phase bus bars with one hand, and another phase with the other hand.
What would this do, i dont want to find out.
Mind you, i have taken non isolated 240VAC from hand to hand before and it didnt kill me...but i could feel my pectoral muscles vibrating!...no pain though....i was trying to press some probes across the 240VAC, and the contacts were corroded, so i had to apply loads of pressure, and then my hands slipped...and zappppppp!

But i guess i want to know which regs apply to us.?..and how can you get a "clause" in those regs so you can do what we want to do (ie 50Hz AC in three bare bus bars)?

What you are proposing is crazy and I hope no qualified electrician will entertain you.

Do the job right or don't do it at all

 

[DPG]

I'm surprised your electrical guy hasn't raised concerns about what you are proposing to be honest.

 

[Strima]

Telephone exchanges, where only instructed technical people were allowed in, made widespread use of 50V busbars.

And a lot of them are still up and running today. I'm not a fan of the aluminium ones but the copper ones look lovely, would look even nicer in the back of my van going to be weighed in... ?

 

[Zerax]

So... treesleef... what's your involvement with this 'art gallery' ? And can you give us a high level synopsis of what you are trying to achieve ? I.e. don't include anything to do with the solution that you have in your head... just the end result of how you want 'things' to look.

 

[Strima]

We will have signs up saying "don't touch the bus bars"

Because people are renowned for reading and obeying signs? What happens if they can't read or their first language isn't English?

Touch safe might be fine for the vast majority of people, but what about someone with unidentified underlying health problems such as a heart condition? Even a few volts and amps could be fatal.

You claim to have had a 240 volt shcok, doubtful it was the full 240 volts as there are many variables such as body resistance, moisture content, type of footwear, flooring type and construction which alter the shock level significantly. To assume that a similar shock to others would be not just stupid but potentially fatal.

Many people refer to BS7671 as the regulations, they're only guidance but are used to help us comply with EAWR.

To be 100% certain that your within the law you must be satisfied that the installation and practices comply with the The Electricity at Work Regulations 1989.

You have a duty of care towards your staff and visitors to your gallery. Don't try and find a get around which will endanger people. Your attitude towards safety is a little blasé to say the least.

 

[Darkwood]

If you cannot design this to be safe at the voltages we suggest then we have what I call a conflict of interest in designing what you have in mind and the limitations imposed on the design by safety and regulation, it simply may be the case you cannot do this the way you want.

I would however suggest other alternative in that you fit a standard mains insulated busbar rail with insulated connectors and tap out of it at various intervals into multiple control boxes that feed smaller arrays of lighting at a safe extra low voltage rating SELF, this will get around the power distribution issue and allow for multiple tapping points where suitable, it also gives a degree of redundancy that a issue of one array of lights will not shut the whole feature down, assuming that the power control boxes tapped off are done in a suitable manor.

You can still achieve a smart lighting set up by linking slave PLC's to a master PLC and have them controlling the lighting, this is by no means cheap but nor is this kind of installation either, I have seen some of the grants out there for art displays and I wouldn't be surprised if you can cover the cost with one.

 

[StephenRowley]

What you are proposing is crazy and I hope no qualified electrician will entertain you.

Do the job right or don't do it at all

I suspect there insurance may have something to say as well.

Just use one of the 240v lighting rail system, like every one else. It'll be cheaper and much safer.

 

[Steviec1]

Thanks...switchgear?.....you mean the switch to the primarys of the 10kW three phase transformer?
Our electrical guy i presume is doing this.
I presume it will be a big contactor with a biggish manual lever and these arent that expensive i hope?

The Art Gallery contains high price exhibits and the general public are not allowed in......only film crews who are under strict supervision. We have staff working in the gallery...and they will be trained not to touch the bus bars so i presume since its a "controlled area" then we will be allowed to have bare 50VAC bus bars as described?

We have a prototype, and we can touch one of the bare bus bars (with isolated 40VAC in it at the moment) when its energised and it does us no harm (mind you we have rubber souled trainers on). Mind you, i've never tried grabbing one of the three phase bus bars with one hand, and another phase with the other hand.
What would this do, i dont want to find out.
Mind you, i have taken non isolated 240VAC from hand to hand before and it didnt kill me...but i could feel my pectoral muscles vibrating!...no pain though....i was trying to press some probes across the 240VAC, and the contacts were corroded, so i had to apply loads of pressure, and then my hands slipped...and zappppppp!

But i guess i want to know which regs apply to us.?..and how can you get a "clause" in those regs so you can do what we want to do (ie 50Hz AC in three bare bus bars)?

Is it me or is the phrase "Can of worms" in use here.
Get somebody qualified to check this, somebody will die!!