Earthing of dockside crane

[lee5]

We have an old dockside crane which was converted from diesel to electric 20 years ago. It is supplied with 3 ph and earth (combined earth and neutral) the IEE would class this as a TN-c system. The crane runs 50 meters either side of the supply point. An inspector has called this into question and has stated that TN-C is only applicable to fixed installations. I have checked out BS2452:1954 but the only thing in their about earhting is "The earthing shall be provided independently of the contact between the track wheels and the rails."

Can anyone help on whether a combined earth and neutral is acceptable in a mobile system such as this or should we have a seperate earth and neutral

 

[lyledunn]

lee5,
I believe your inspector to be right.See Bs7671 546-01-01.
In any event loss of neutral in such a system results in major hazard. Electricity Supply Regs demand specific features for the TNC part of the common TNCS system.
Because TNC systems are uncommon in UK, for reasons cited, it would be difficult to defend any action brought under EAW Regs should, God forbid, an accident happen,especially now that inspector has highlighted possible problem.
Reg 511-01-02 of BS7671 may leave the designer or other responsible person in the unenviable position of trying to justify that the your crane installation is as safe as it might have been using another recognised system. Regards,

Lyledunn

 

[welshwizard]

As you say lee5 TN-C or PEN conductors are not very common

Do you need a neutral if the motor is a 3 phase balanced load or is the neutral required for a control circuit function?
RCD's will not operate with a PEN conductor on their upstream side under earth fault conditions
BS7671 has quite a bit to say about PEN conducitors you do need authority for it from your electricity supplier. In any event it is designed to cover"fixed installations" which is why the Inspector made his comments

You may be ok if you have your own supply from a private generating plant - transformer or converter.
Connected in such a way that there is no connection -except
for the earthing connection with the general public supply
See BS7671 Reg546-02-08 and others for more details

As lyledunn comments tread carefully!