Harris Institute of Technical Training

On a padmounted transformer where would you put your voltage tap since the ct sits at the back of the bushing and there is no where to install it before the ct. When install after the ct the consumer is being charged for power to run the meter. Even though this amount is small it still shouldn't be consumer's responsibilty to pay to run utilities meter. Could you install on the pad and run backward
s through the ct?

My first step was to look in my Meterman’s Handbooks (9th and 10th editions) to see if I could find a section that might address doing this and I couldn’t find anything there. You are right on in your thinking that the correct way to make it so the customer is not paying to energize the meter is to run the meter potential coil leads backwards through the CTs. When you do this then the energizing current for the meter potential coils is subtracted from the total current the CTs are measuring. As you do this, you need to make sure that you are running these wires correctly so that you don’t add the energizing current a second time.

I agree with you, that when the CTs are placed on the bushings of the transformer, there really isn’t any way to connect the voltage tap wires in front of the CTs so that the customer doesn’t pay to energize the meter potential coils. Connecting the potential coil(s) voltage taps after the CTs will make it so the current needed to make the meter potential coils operate is part of the full current measured by the CTs. Running the voltage tap wires backwards through the CTs will subtract the energizing current for the potential coils. If you have questions on the way to do this correctly, I can e-mail you a pdf file showing the proper connection diagram (send me an e-mail at Metergod@live.com).

On an interesting side note, I removed a 120 volt potential coil from a three element form 9S electro-mechanical meter several years ago, just to measure how much current is needed to energize it, I found that this 120 volt potential coil was consuming about 50 volt amps which is .42 amps (@ 120 volts). Because potential coils from these old electro-mechanical meters are for the most part mini voltage transformers, the energizing current (.42 amps) is mostly an inductive current with a small amount of watt or resistive current. So, with this said, as you run your voltage tap wires backwards through the CTs you will be subtracting more inductive current than resistive or watt current from the CTs.

Also, If anyone else is interested in receiving the pdf drawings (as noted in the above answer), please feel free to e-mail me.

Can you send me the PDF drawing?