Generator Theory

[RegElec]

I have a question for the theory boffins.

A recent discussion resulted from the question as to why would a 3 phase generator (portable stand alone) that is connected to a container office as the only source of supply require an earth spike in the ground. The feed from the generator is via a cable (+-6m) to the container office single phase DB.

Any theories on this? Unfortunately I dont have any further details on the generator regarding winding configuration etc but there is a built in main switch, earth leakages (4p + 2P) and single phase circuit breakers.

[ACEsterhuizen]

Aahhh Its easy.... just follow the regs... it "explains itself??"

[RegElec]

The regs only deal with a generator as alternate supply with the exception of a single phase generator with V-O-V configuration. I am more interested in the theory as a stand alone supply on a three phase generator with regards to an earth spike

[Dave A]

The objective is to ensure "correct operation of the protection devices, irrespective of the source of supply" (a requirement of 7.12.3.1, which deals with Neutral bar earthing).
Essentially to achieve this you need to prevent the neutral from floating off earth, and the earth to maintain a voltage as close to ground as possible (among other things). Hence in your example you would require the earth and neutral to be bridged as close to the power source as practical, and the earth to be grounded via an earth spike.

In the test section we have:

8.7.6 Elevated voltage on supply neutral
With the main switch off, measure the voltage between the supply neutral and any earth external to the installation.

Essentially this requires corrective action if above 25 Volts and disconnection if above 50 Volts.