Neutral Ground Connection

Are you using a 3 pole or a 4 pole transfer switch?

As you have mentioned that this is an educational project, let's work from the premise.

To answer your question, as Wareagle has suggested, you must first decide upon the nature of the transfer switch.

So... why is that? What do you think could be impacted by the nature of the switch?

I understand that you are talking of the AMF system & only one source of supply will be feeding the load at a time. Normally for this kind of installation it is recommended to have four pole switchgear, so that neutral of both i.e. utility & DG remain separate.

If you have to run both the system in parallel for some time, then we have to ensure that during parallel operation only one neutral is connected to earth. Therefore we have to use there pole swithgear for three phase & separate contactor for neutral of suitable rating.

In any case both the neutrals should not be connected to ground as this will result in the neutarl circlation current.

Regards

D K KALIA

Chandigarh

First of all the neutral connection to ground depends on the type of grounding scheme you want to use High impedance,low imedance,solid grounding or you dont want to ground at all.All depends on the fault currents levels and protection settings,generator insulation level etc.Because if you dont connect the neutral to ground then i n case of a ground fault the voltage in the un faulted phases may rise to phase to phase voltage level..also if you do dont ground both supplies then use a 4 pole transfer switch and if you want to operate in parallel then you have to use some sort of sync relay as well...if you dont connect both gen. to ground then its ok if you dont have to operate the two supplies in parallel but if you have to operate in paralle then you cannot ground both gen or circulating currents will flow...

Do not hook the generator neutral to the power company's neutral. If you do this then current returning from loads powered by your generator will be able to travel into the power company's neutral conductor which would be hazardous to utility workers because they have no way to shutdown your generator when they need to work on transmission lines.

Based on your description, your "transfer switch" (US term) must completely disconnect all current carrying conductors leading into your distribution panel (breaker or fuse box) from the utility lines and replace the connections with lines from your generator so that your distribution system has no tie-in with the power company's.

If you are in the USA then do not connect the neutral to ground at the generator because the US NEC requires the neutral to be bonded to ground in the first electrical distribution panel after the source not in the transformer or generator. (I have been told that this is the opposite of the European Standard, but you should verify this because I am not certain.) Whatever you do, do not bond the neutral to ground at both the generator and at the distribution panel. If you do this then electrical current will enter into your grounding system which could shock you when you touch the housings of grounded equipment.

Cutting off the generator should cause all electrical devices supplied by the generator to shut down instantly (like turning off the Main disconnect). The transfer switch should not be designed to reset back to utility power automatically when the generator is shut down. The transfer to generator power is usually automatic, but the transfer back to utility power should require human intervention (your local standards may differ, but in my opinion this is best for safety). This is usually accomplished by requiring a manual handle to be switched back to position or with an electronic reset button. This is to help ensure that all of the electrical devices powered by the distribution panel are set to "safe states" (usually OFF) before power is restored.