Generators

[electronicsuk]

Why would an earthed frame cause problems if it were bonded to neutral? Surely it would only be an issue if, as already stated, the earth were centre tapped rather than electrically seperated and truly 'floating'.

 

[NotHimAgain]

If the frame is connected to the centre tap on the winding - as it often is on small generators - you will short circuit half the winding if you earth one of the outers (what you are calling a neutral) Try it - the generators are quite cheap these days .

 

[electronicsuk]

If the frame is connected to the centre tap on the winding - as it often is on small generators - you will short circuit half the winding if you earth one of the outers (what you are calling a neutral) Try it - the generators are quite cheap these days .

Re-read my post - that's pretty much what I said. However, it wouldn't cause a problem with a true, electrically seperate, floating earth.

 

[NotHimAgain]

I said

Please note that any attempt to earth the 'neutral' end of the winding will end in tears if the generator earth connects internally to its frame . icon_eek.gif

I think we are talking at cross purposes here don't you.

 

[ericmark]

If the frame is connected to the centre tap on the winding - as it often is on small generators - you will short circuit half the winding if you earth one of the outers (what you are calling a neutral) Try it - the generators are quite cheap these days .

I have also come across generators with centre tap. Designed for duel voltage 110/230 with two winding either parallel (110) or series (230) one winding being centre tapped to earth. OK in 110 format but in 230 volt mode gives 57-0-173 volt.

Any neutral earth fault with duel pole MCB should trip the MCB but it means what every is supplied needs duel pole protection and many items have fuses on the line supply only.

With one I had I removed to 230 volt option if 230 volt was required they had to use step up transformer.

However the voltages you report makes me think more likely earth and neutral are not connected and where a single item is used this is not a problem and is used for safety with shaver sockets etc.

However where multi items are used this will not work.

Section 717 covers this and shows a connection between earth and neutral and I would consider any generator with centre tap unsuitable for purpose and would not use it.

See Electrical safety of independent low voltage AC

Direct link to PDF document here

 

[Spark123]

I suggest you read BS 7430:1998 (18.2.5) and search for the HSE guidance on this subject.

Please note that any attempt to earth the 'neutral' end of the winding will end in tears if the generator earth connects internally to its frame .

Just read BS7430:1998 18.2.5 - appears to say what I said before
Small generators are often floating, if using RCDs you need to have connections upstream of the RCD between an unswitched pole of the generator, the protective conductor and earth thus this pole becomes a neutral.
That wouldn't work if your generator is centre tapped hence the reason I said about Meggering the winding to the chassis/earth connection to ensure it is actually floating. If it is down in the ohms range then it is not a good idea! From experience, most small generators are left floating - certainly mine is.

 

[NotHimAgain]

For the benefit of those without a copy - the second paragraph of 18.2.5 says:

It is accepted that small single-phase generators can run satisfactorily as floating systems, i.e. without the winding connected to the frame or earth. The generator frame and enclosure should be bonded to all exposed-conductive-parts of the load equipment, using a correctly terminated protective conductor in the connecting cable.

Now we all appear to be at cross purposes here

You did indeed talk about floating systems with no earth connection on the winding and no reference to the frame. You correctly said that an RCD would not work under those conditions.

I said

With a centre tapped system that is floating with respect to true earth an RCD can provide protection but not against shocks to true earth.

The exposed-conductive-parts of the installation (food van) are connected as usual to the earth terminal of the generator. Any faults within the installation between either outer conductor and the exposed-conductive-parts will operate the RCD.

What will not happen is protection against faults to true earth. This is because we have not referenced to true earth. Should a 'first fault' occur that does reference the system to true earth a hazard will occur.

This is different - the winding centre tap is connected to the frame.

 

[Spark123]

How do you end up with a shock to true earth, given that the system has no reference to earth?

 

[NotHimAgain]

You would only get one if first fault had already earth referenced the system.

It is accepted that in very small installations this is unlikely. The best use of this is for a single item such as a power tool. The food van can be stretching things a bit.

Still the next time you visit the kabab van on a Saturday night it probably won't be an electric shock that kills you

 

[ColJack]

youst don't lean on the counter whilst barefoot and you should be fine