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Protection at transformer and generator
6 Posts
Is there a requirement for a protective device at both ends of a cable(s) between a dedicated transformer and large fixed gen set operating in parallel with the grid, I  vaguely remember seeing it in 7671 but cannot easily find it using the online Vital Source Bookshelf. (I have assumed that all G99 etc. protection is taken care of by the gen incomer.) 

Reg 434.4 allows for omissions against fault current under certain circumstances, one being - For a conductor connecting a generator, transformer, rectifier or battery to its associated control panel.
3 Replies
Good Morning,
Schema Attached,
I hope this helps you understand.
In the drawing, a transformer appears on the pillar feeder counter in a new structure with a grounding ground.
A mass cabinet feeds the main board which feeds four cable sub-boards as shown in the drawing.
Assuming that the planner chose TNCS as the method of protection in the structure, the sectional area required for the reset conductor in the drawing is: "70 mm copper conductor"
6 Posts
Hi Benyamin,

Thanks for replying however i don't think the answer is relevant to the question. 

726 Posts
You don't need a protective device at the feeding end of the transformer supply as that can be taken care of at the receiving switchboard incoming device (usually with REF) and by reliance on the HV circuit breaker on the primary side of the transformer for short circuit.

You could have the same arrangement with the gen set as it's in parallel with the LV supply  - again, typically with REF looking back into the set windings and with a through fault short circuit tripping the set

You ideally need the switchboard circuit breakers to be set up for directional overcurrent as well - you don't want, for example, the LV mains supply feeding back into a gen set winding short circuit.

It would be more usual to have a gen set with source and receiving end protection (if you also need an island mode operation) and receiving end with the mains and basic reliance on the HV breaker

I think you are mixing up the requirements for control functions to those of the "raw" power.

Ideally, you would have protection on both directions for both supplies  - the G59 protection being an add on via a separate relay to the mains CB along with any reverse power export criteria.

Or have I misunderstood the question ?





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