Form 3S Meter

[Jason_R]

Hello all,

I am trying to find out more information about how a Form 3S meter works. I know that this is a transformer rated meter, single phase, three-wire. My question is how does the single CT correctly measure the current running through the service? Please also explain why you would use one-half the CT ratio when determining the meter multiplier.

Thanks for any insight.

 

[stevenal]

The CT is connected so that it will add the negative of one leg to the positive of the other. Halving the multiplier provides the average of the two leg currents. Voltage is measured leg to leg. This meter form violated Blondel, but provides good accuracy when the two leg to neutral voltages are balanced.

 

[waross]

Further to Stevenal's explanation; The standard residential meter also uses the line to line voltage and makes a similar assumption that the line to neutral voltages are equal. Unbalanced loads cause slightly unbalanced line to neutral voltages, resulting in slight metering errors.
The voltage drop on the neutral conductor in a 120:240 Volt circuit expressed as a percentage of the line to line voltage is approximately equal to the metering error. eg: For a 120 Volt load on a 120/240 Volt circuit, a 6 Volt drop on the neutral = 6 Volts / (240-6 = 234 Volts) = 2.56% over metering.
Residential metering has been this way for probably over 80 years. The error is small enough that generally no one notices.
There is also a non Blondel solution to metering four wire three phase that uses the CT secondaries in delta and assumes that the three phase voltages are equal. This particular solution will also work with only two CTs if it is physically possible to pass the conductors of the third phase through both CTs. The few times that I have used the non Blondel solution for three phase metering I had too many large parallel conductors to pass the third phase through the other CTs and used three CTs.

Bill
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Jimmy Carter