two wattmeter-Earthing

Hello gova1467

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Kind Regards....

Dear sir

what is the input quantity required to energy meter ?

Ans:voltage from PT and current from CT considering HT service.

Then, PT secondary (any one phase) is earthed purposely leaving neutral in floating.

My question is '

1)is there any advantage to meter if any one phases of PT secondary is earthed.

2)what are the factors we have to consider for determining the rating of fuse used in PT secondary side?

thanks and regards

gova

GOVA, the reason for earthing is always safety. In the USA every circuit lower than 300 volts is required to have one grounded leg, even in metering. This enhances the function of protective devices. You will also find that the CT secondary is referenced to earth at one point; again for safety.

What you have described is a corner grounded delta secondary, in which case the "neutral" , probably a center tap of one of the PT's, if also grounded would cause a catastrophic short circuit and so is left purposely floating.

For the fusing, the purpose is to protect either the wiring or the devices in case of a fault, which could involve the primary voltage available and should be so rated. Current value of the fuse is determined by the meter circuit to protect the meter.

May I inquire why you mentioned "two wattmeter" in your question?

Dear CJMcGill,

Phase earthing is done only in two wattmeter method connection.The neutral is kept floated and not connected to meter.

But in three wattemeter method,neutal earthed and connected to meter.

This is we followed in our Electricity board.

Pl share ur views.

Regards

gova

I now recognize the terms 'two wattmeter' and 'three wattmeter' to be the equivalent of 'two element' and 'three element' meters in our terms. Additionally it is clear that the voltages available to you for metering purposes as stated are the secondary values of Potential Transformers, and the currents the metering will see are the secondary currents from Current Transformers. Both of these circuits are required to be grounded, or 'earthed', at one point only, for safety purposes.

Determination of whether to ground a phase or the neutral conductor will be dictated by the voltages and configuration (Wye or Delta) of the meter circuit. Whether to run the neutral to the meter or not is dictated by the voltages required by the meter and the form of the meter.

Since there are 45+ meter forms available, it is clear that your Electricity Board has chosen to keep the choices simple, which can be a good thing.

dear CJMcGill

Thanks for ur immediate response.

regards

gova