Neutral-earth bonding

[dor]

Sorry if you have answered this before.  I've used the search facility, but am even more confused!

I have an L-series Powermaster 1800 PSW inverter.  This has run anything I have tried on it.  Whilst doing some general maintenance I thought I would check the neutral-earth bonding.  However what I have is N-L 235V,  N-E ~150V, L-E ~150V.   I.e it would appear that this is effectively centre-tapped with no neutral-earth link.  In some of the items that came up in my search was one saying that they had the same inverter with a N-E link,  and Gibbo (Oh where is he now?) gave:

Those that bond neutral/earth that I am aware of are....

All Sterling inverters over 500 Watts.

Mastervolt Dakar.

Mastervolt Mass.

Trace truck power.

Trace SW series.

Trace XW series.

Heart Freedom series.

Heart EMS series.

All Victrons.

All PowerMaster.

However, apart from my observations given above, the Powermaster User Manual says:

230V models: There is no connection made inside the Inverter from either of the line conductors (line or neutral) to the earth ground.

It's an excellent inverter but unfortunately Powermaster gave up the ghost several years ago.  Can anybody enlighten me on what I should do to ensure that the RCD will trip in the event of a leak to earth?

[WotEver]

If it is centre-tapped earth then it doesn’t require (indeed you mustn’t even try to fit) a neutral-earth bond. It is perfectly safe as is and the RCD will operate as normal. 

The test to 100% confirm that it’s genuinely centre tapped would be to connect a 60W lightbulb between earth and ‘live’ and then again between earth and ‘neutral’. If the bulb lights equally on both then it is indeed centre-tapped. ONLY DO THIS CHECK IF YOU KNOW WHAT YOU ARE DOING AND FEEL CONFIDENT TO DO SO. You could kill yourself playing with mains electricity. 

Tony

[nicknorman]

Yes the point is that a multimeter doesn’t put a significant load on the output to pull the line under test, to the earth line. At Tony says, you need to put a reasonable load between earth and either L or N.

if it is indeed centre tapped, an RCD will still work fine.

Edited April 15, 2018 by nicknorman

[dor]

Thanks for your reply  Tony.

I'm still a bit confused (and I thought I had a good grasp of boat electrics).  We've seen so often about how it is essential to have this E-N link to make sure that the RCD will trip under a fault condition, so how is it ok if it is centre-tapped?

[nicknorman]

40 minutes ago, dor said:

Thanks for your reply  Tony.

I'm still a bit confused (and I thought I had a good grasp of boat electrics).  We've seen so often about how it is essential to have this E-N link to make sure that the RCD will trip under a fault condition, so how is it ok if it is centre-tapped?

Because it means the supply has a fixed relationship to earth.

Consider an inverter that has a fully floating output, you can touch either the nominally live or the nominally neutral connection and not get a shock. Which might sound a good thing but it means that there can be a long standing fault connecting live to earth, or neutral to earth and nothing will trip. Then if you accidentally bridge between earth and the other terminal, you die.

If on the other hand you have a centre tapped supply, if either the nominal N or L supply develops a fault which connects it to earth, the MCB trips immediately. So you can’t die due to a long term fault like that. If you accidentally touch L or N and earth, the RCD trips and stops you dying. So if you really want to die, the only way is to touch both L and N at the same time, preferably with your tongue or some other such damp protuberance.

[dor]

Thanks Nick.  This is beginning to make sense to me now.

Not sure what damp protuberance you might have in mind, but I will keep mine well away from any electrical socket...

[Chewbacka]

Inverters can be a bit 'strange' the one I have has a couple of capacitors to make it act as centre tapped but the manual also states that if you prefer it is acceptable to EN bond the inverter - which I have.  So just testing with a multimeter may not tell you exactly what is going on.  So if you KNOW what you are doing the light bulb method above is a good check, but if in doubt leave it alone.

[BruceinSanity]

You can buy a basic socket tester with RCBO test option for not much money. Wouldn't one of these establish whether the system is safe? They work by putting a high value resistor across L-E.

[nicknorman]

27 minutes ago, BruceinSanity said:

You can buy a basic socket tester with RCBO test option for not much money. Wouldn't one of these establish whether the system is safe? They work by putting a high value resistor across L-E.

No, since a centre tapped system only has 115v between L and E, the resistor value is too high to get the 30mA or so necessary to trip the RCD.

[WotEver]

59 minutes ago, nicknorman said:

No, since a centre tapped system only has 115v between L and E, the resistor value is too high to get the 30mA or so necessary to trip the RCD.

Ooh now, where’s GrahamM when you need him  so we can have an argument about his favourite subject.