Generators

[Albert]

Hi,
How would you measure the RCD trip time and Idn in a food van' supplied by a 4kW generator. There is no Rod, the van chassis is the earth. The reading between earth and live is 105V and between live and neutral 216V.

 

[nozspark]

Surely the neutral & earth are tied at the generator????

Also it should really have a rod!!

 

[Albert]

Surely the neutral & earth are tied at the generator????

Yes but it does not help with the RCD tests, I can do the Zs (EFLI) using 2 probs but this cannot work with the RCD test, or I am wrong?

Also it should really have a rod!!

This is one option (see section 717 fig 717.1 page206) but you do not have to have a rod, you can use the van chassis as your earth ((this is done by the manufacturer), and this is the case I am talking about

 

[electronicsuk]

Sounds to me like the earth is floating, N and E probably aren't linked at the genny, so you should probably investigate this before going any further.

 

[Albert]

Sounds to me like the earth is floating, N and E probably aren't linked at the genny, so you should probably investigate this before going any further.

Thanks, what in your opinion is the best way to test this.

 

[electronicsuk]

Sounds to me like the earth is floating, N and E probably aren't linked at the genny, so you should probably investigate this before going any further.

Thanks, what in your opinion is the best way to test this.

You've almost confirmed it already with those voltage readings. If N-E were linked then the voltage between the two would be negligible, which doesn't seem to be the case. It could be a break in the earthing elsewhere on the trailer, but the generator would be my first suspect.

A simple way to confirm would be a continuity check between the E and N at the generator itself when not running. Small portable generators rarely have the two linked together during manufacture. If a link does need to be fitted then you could consider making the connection inside the plug (I assume it's a commando socket?) and attaching a suitable warning notice.

 

[Albert]

Just talked to the customer and he is going to buy a new generator, hopefully it will solve this part of the problem. I still have the RCD testing issue, do you think that if the earthing in the generator will be ok it will allow to run the RCD test?

 

[electronicsuk]

Just talked to the customer and he is going to buy a new generator, hopefully it will solve this part of the problem. I still have the RCD testing issue, do you think that if the earthing in the generator will be ok it will allow to run the RCD test?

I don't see why not. Obviously the testing would have been pointless before with a floating earth, but with N-E linked there should be absolutely no problems with incorrect operation of the RCDs when on generator.

 

[Albert]

Many thanks for the advice, being honest (probably it is obvious) although I have done many domestic and commercial inspections and tests, this is the first time that it involves a catering van. I like to prepare before I do something that i have no experience and you were very helpful!
Thanks again.

 

[NotHimAgain]

Many small generators have a centre tapped secondary winding with a mid point earth. The supply is taken between the outer conductors (neither of these is a real neutral).

These generators are often operated as IT systems, and are often floating with respect to earth. This is OK on small installations provided that a 'first fault' does not occur. There is HSE guidance on the use of these system.

Whatever you do - do not connect the earth to what you think is a neutral - that will go bang

Most RCD testers will not work on these systems but the RCD does provide some protection, especially if a first fault occurs.

 

[Albert]

This inspection requires a periodic inspection report, what do you put in the part of the RCD test results?

 

[flameport]

You can test RCDs on IT systems by connecting the L and N leads to the output terminals of the RCD as usual, but connect the E lead to the incoming neutral before the RCD. This will create the required imbalance to trip the device.

However if the supply is not referenced to earth, the RCD isn't going to be much use anyway.

 

[NotHimAgain]

If it is centre tapped you must not connect neutral and earth together - you will short circuit half of the winding.

 

[Spark123]

Often small generators are "Electrically Separate" which means that there is no reference to earth to the winding of the alternator (often referred to as floating).
Sticking a megger between the winding and the chassis/earth of the generator will confirm this - if there is MegaOhms of resistance it will be floating.

If it is not a Terra system (as in Tn-s) then RCDs do not afford protection bar acting as an On/Off switch!!
First faults are not easilly detected - there is a great reliance on the equipotential bonding of any attached equipment via the earth in their leads, double poled protective devices should also be used.
A 2nd fault on the opposide phase to the 1st (both faults to the eq bond) will drive all fault current via the bonding to each other. If this bonding fails there can be 240v between exposed conductive parts.
This is why it is preferable to only run one appliance on a short system from them.

Instead of relying on a Separate system it can be turned into a TN-S system. You will need to provide the means of earthing via an electrode and tie the Neutral side to the CPC and Earth. (Make sure it is floating first tho ) Any faults can flow back via the CPC or the electrode hence RCDs can be effective.

 

[NotHimAgain]

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.

The judgement is - how likely is the fault. We have a vehicle with rubber types and a small electrical installation .

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 .