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?

 

[westward10]

50vac is considered to be safe electric shock wise however it can still subject you to pain and possible injury, maybe not an electrical injury but from the result of falling for example. You must provide basic protection.

 

[Pete999]

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?

Depends on the rating of the BBs

 

[snowhead]

Way too many questions.
I'll try one.
Are these Bus bars in a building in the U.K? (I know it says Southampton on the O.P profile)

 

[Lucien Nunes]

Whatever the voltage, there is a risk of burns or arc flash if the source impedance is low enough. Even at say 5V, a high current supply could cause life-changing injury if metal jewellery shorted the busbars together. As already stated, if these are a supply of energy, rather than perhaps a signal source referenced to the AC mains through a high impedance, then it will not generally be acceptable for them to be exposed. What is the application?

 

[treezleef]

The application is an "art" gallery displaying multiple exhibits of interest.
Also, we are in UK.
So if we keep the bus bars below 50VAC then we will be OK? They are after all, isolated from the mains.

 

[DPG]

Why not just stick to 12V? Is it just to light lamps? Give us some more info.

 

[pc1966]

As above, 50V AC is considered "safe" for situations where contact is not expected and water is not present, but high current can cause serious burns. For at-risk areas like swimming pools it is usually taken as 25V or less.

If it is simply for some art display then go for 12-24V or similar but make damn sure it is current limited to a could of amps at most, that should still allow some reasonablt amount of lights or small motors to be run from it.

 

[freddo]

I'm intrigued by the need for 3 phase?

 

[DPG]

I'm intrigued by the need for 3 phase?

Same here. The OP seems reluctant to give much info.

 

[ipf]

Sounds dicey to me. Not enough info and whiffs of DIY.

 

[StephenRowley]

What do mean for a art gallery display?

Is it for lighting?
Power items in Said display?

I'm intrigued

 

[ipf]

What do mean for a art gallery display?

Is it for lighting?
Power items in Said display?

I'm intrigued

Maybe it's just available...can't think of any other reason.

 

[ElectroChem]

I'm thinking some kind of silly interactive 'modern art' exhibit. "Grab the busbars, it's perfectly safe!" No thanks, I've seen the autopsy reports from electrocutions.

 

[Wilko]

For what it’s worth, Class lll appliance design might be of interest. A suspended bare cable light system might be designed for this class and they are usually 12V, supplied by certified isolation transformer etc.

 

[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)?