Dangerous voltage on bare bus bars?

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opampsmoker

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Hi,
We will have isolated three phase bus bars. Fully Mains transformer isolated. (50Hz, 240VAC phase to neutral)
We're going to step down the 3 phase mains to the bus bars with a 50Hz mains 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)
 
I spent a good few years in a classroom learning all this.... I suggest you do the same.... Even 50vac can bite in certain circumstances..
 
Thanks, but our situation (bare bus bars) is so unique that its impossible to tell what the regs are. Maybe we can get away with 50VAC on bare bus bars as long as we ensure that everyone entering the room is told not to touch the bus bars?...or we have a big sign up warning people not to touch the bus bars?
 

it doesn't matter what we think is safe, it matters what your local codes, regulators, and insurance carrier thinks is safe. Dry skin, wet skin, wet fingers/dry skin, salty water moisture vs distilled water moisture, ... it also depends if the complete circuit goes from one finger to the next, or a finger to boots to ground, or one hand and across a humid skin surface to the other hand, or one hand though your chest/heart and out to the other hand. Unfortunately, you don't get to pick the circumstances or path of current flow - Mother Nature picks and her answer may be different any given second.
 

Don't use bare bus bars - easy answer.

I seem to remember vaguely that at one time the USA used to use 60V mains, on farms etc. (rather like we use 110V transformers for builders tools now), and from what I remember people managed to get killed even on 60V mains.

If there's some particular reason you've got to have bare bus bars - then I would suggest sticking to 12V, as used in a car battery.

However, bear in mind, some bright spark is likely to stick a piece of metal across them - BOOM!!!
 
Beau Schwabe said:
We just completed an industrial application for a Client ... Think roller coaster where each carriage is powered on a 100 foot track. To keep certification inspectors happy, the Maximum allowable voltage for the system was 48Volts DC or AC
Click to expand...

A friend of mine worked (and perhaps still does?) at a Toyota car factory - and the cars move along on overhead tracks powered by three phase mains, connected via three rails beneath them - the rails are vertically above each other. Now these tracks wear our, and break off, and the upper part falls on the one below - you can imagine it doesn't help things much - boom/flash. I couldn't help thinking that's rather a design flaw?.

One of my friends job was replacing the tracks, and they were getting low, so tried to order some more - they were told they had to come from Japan, with a six month lead time They tried pointing out the exact same part was available in Europe, and they could be delivered tomorrow - but no, they had to come from Japan.

According to my friend, they were within two days of teams of men pulling the cars round the tracks with ropes when the supplies from Japan eventually arrived.
 
The limit in schools for PA and intercom speaker signals is 25VAC, in case a student is crawling around in the ceiling.
 
You can bet that there will be regulations that cover what you want to do - although in my experience regulations are always pretty opaque. As other have said, it depends rather on where in the world you are.
In the UK and Europe, I think what you want to do may be covered by SELV (seperated extra low voltage) or PELV (protective extra low voltage), which would limit the voltage to 50VAC or 120VDC. Again, as others have said, it will depend somewhat on the environment and the people - if they are not the general public, you may get away with more.
My suggestion, though, would be to read up on SELV and PELV, and equip yourself with a regulations book if you possibly can.
 
Beau Schwabe said:
the Maximum allowable voltage for the system was 48Volts DC or AC
Click to expand...
When the government is involved, and they all the time are, 48 volts is the answer. It does not matter what voltage I think is save.
but
This depends on the environment. Dairy farms and operating rooms have very different laws. (wet environments and possible contact inside the body) A dairy cow comes unglued at just a couple of volts because the current flows through the milk and inside. She is also standing on a wet floor.
 
What was the little blue book called that had all the U.K. regulations in? It was something like the 16th edition and was pretty much the electrical bible back in the 90s.

Mike.
 
the only open buss bars I am aware
Pommie said:
What was the little blue book called that had all the U.K. regulations in? It was something like the 16th edition and was pretty much the electrical bible back in the 90s.

Mike.
Click to expand...
it was called THE QUEEN'S BOOK OF ELECTRICAL CODES AND REGULATIONS.
 
It was BS 7671 "Requirements for Electrical Installations. IET Wiring Regulations" 16th edition which has now been superseded by the 18th edition. This will have the current regulations. Anyone got a copy?

Mike.
 
Here is what I would do.

Get a 2:1 transformer who's primary can handle the fault voltage, but connect it to your normal 230VAC phase to neutral voltage. The secondary will be about 115VAC which is within the input range of most PFC switch mode supplies. If you ever do loose the neutral, the voltage to the power supply, even if it were to double, will still be within the operating range of the supply.

If you put the transformer in front of each PSU, or group of PSUs, your building power distribution is completely standard.

As for the transformer, it could be an autoformer, which would be cheaper and smaller than a full isolation transformer of the same power.
 
The important question is: What voltage does the ultimate load require? That's the important design criterion. Whatever voltage you end up using to meet that requirement, you may or may not require a physical guard around the busbars. Designing a busbar system so that people can touch it, but without regard to the end use, is utterly ridiculous.
 
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