TT and Isolated power supply question

[Matt]

Hello,

This might be a daft one, but hell. I recently spoke with one of the qualified sparks on my site. He was installing an ISP (isolated power supply) cabinet for a bunch of circuits. As I never seen one of these before I became interrested and asked him about what it was and how it works.

He said two things:

1) that the ISP keeps certain circuits operational even under fault conditions, at the same time indicating there is a fault. It does so because for example in a hospital/data centre environment, there are certain machines which can not be switched off een if they develop a fault, because that could put peoples lives at risk or do damage to some systems handling valuable data.

2) he told me that because the system provides power to the circuits through an un-earthed transformer, then unless you touch both the line and neutral (he actually said that there is no neutral but two line) together, there is no chance of you becoming zapped...

I mean what???

I have some questions about this because what he said bassicly wrecked my "understanding" of how current flows in a circuit and how safety devices operate. Not that my grip on these was particularly strong to begin with.

SO I have some questions that, I hope, someone here can give some answers to.

1) If a fault occurs on a device powered through an ISP, this is a 230V system, won't that in time cause the device to... just fail at some point? Go up in smoke if you will?
What sort of safety devices are used on these?

2) I completly do not understand how you do not get zapped if you grab any part of such a circuit. I mean, the floor is still earth, and the main transformer to the ISP has the same potential difference of 230 between its lines, so why is there no shock?
And then, if an earth path does not exist, I would imagine that by touching a live circuit you essentialy mae yourseld become a radial or a spur. Is there really no consecuence in touching such a circuit in one place?

I honstly do not get this, and the guy wasn't really speaking good English so it made things even more complicated.

Anyone? Thanks

 

[westward10]

I suspect they are referring to an IT system not TT.

IT system: Unearthed systems for maximum availability

The underestimated system type offers many advantages for all types of installations. All information relating to the IT system in one place.

www.bender-uk.com

 

[davesparks]

By isolating the supply from earth via a transformer you cannot have any voltage between either line and earth, and thus you cannot have any current flow of you connect them together.

Any fault which causes a connection between one of the lines and earth would be described as a 'first fault', this will not result in any fault current flowing. This does however cause the output to no longer be isolated, it effectively makes the faulty line become a neutral, and this is detected by the monitoring devices.
This allows the fault to be located and repaired.
If it is not repaired then if a second fault occurs then it will cause a protective device to operate and disconnect the circuit.

Isolated power supplies don't stop all faults from causing the supply to trip, if an overload occurs or if a fault between the two lines occurs then protection will still operate and cause it to trip out (I think, unless they've got some very clever system I'm not familiar with)

 

[dmxtothemax]

QUOTE - 1 - the floor is still earth
QUOTE 2 - the system provides power to the circuits through an un-earthed transformer,

That's why !

If you touch a live wire feed from this un grounded transformer, then how is the current going to get back to where it came from, the floor is NOT grounded remember, the only place this curremt is seeking is the return side of the ungrounded transformer, which is NOT available !

 

[DPG]

Have a look at the 110V (55-0-55V) isolation transformers used on construction sites.

 

[davesparks]

Have a look at the 110V (55-0-55V) isolation transformers used on construction sites.

If you mean the usual yellow transformers then they aren't isolation transformers, the earth is connected through to the output centre tap.

 

[DPG]

If you mean the usual yellow transformers then they aren't isolation transformers, the earth is connected through to the output centre tap.

But the 110V is brought out from the 'outers' of each winding - it isn't referenced to earth.

 

[davesparks]

But the 110V is brought out from the 'outers' of each winding - it isn't referenced to earth.

Yes it is referenced to earth, the output centre tap is connected to earth.

 

[DPG]

Yes it is referenced to earth, the output centre tap is connected to earth.

I might be mistaken then - I thought that site transformers had to use isolating transformers in them.

 

[Lucien Nunes]

Yellow site transformers are isolating in the sense that the primary and secondary windings of the transformer itself are sufficiently insulated from one another to provide electrical separation. However, to ensure that no voltage >55V exists to earth anywhere downstream (to make it reduced low voltage and mitigate the most likely shock path) it is necessary to earth-reference the secondary centre-tap, normally using the earth from the input side. Hence the input and output terminals are not isolated once the transformer is installed in its yellow box and wired up.

OP: DMX makes the point clearly in post #4. You might be used to thinking that 'earth' will always take current away e.g. from a shock path, perhaps on account of it being the size of a planet. But in fact it's just another conductor leading back to the source of supply, usually the substation transformer where it's connected with a king-size earth electrode, courtesy of the DNO. If not connected, the planet no more forms a circuit than a piece of wire with one end taped up.

This then raises the question, why public supply networks are earthed and not isolated. There are a couple of reasons but a clear one is to do with persistent faults. An IT system is only isolated as long as no 'first fault' exists. Because a first fault won't make itself known by things stopping working, anyone operating an IT system must keep an eye on the earth fault indicator. When it lights up the fault has to be found and fixed to keep the system safe overall. On a public supply, you can't control what other people do with their installations. If someone down the road has a fault but doesn't fix it, the isolation is spoiled for everyone else. Now, the whole system is 'earthed' via the fault but not very well and not at a known point, so the loop impedance is likely to be terrible and even the polarity is unknown. A TN system with ADS, as used on normal public supplies, doesn't allow other people to get away with such laziness. When someone has a first fault their breaker trips and disconnects them, leaving everyone else safe.

 

[DPG]

Thanks for clarifying that Lucien. Must admit I'd forgotten all about the centre tapped earth. Davesparks - apologies, you were correct.

Standing in a non-earthed and non-conducting corner as we speak.

 

[Timbo]

Thanks for clarifying that Lucien. Must admit I'd forgotten all about the centre tapped earth. Davesparks - apologies, you were correct.

Standing in a non-earthed and non-conducting corner as we speak.

You must feel so isolated..... ?