Switchboard Meter Readings

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The individual meters should add to match the main metering in kWhr, with slight differences that will balance out over time. The same is true for kVA or any other metering, but not because it accounts for power factor differences. As long as the main metering and all sub-metering is being done correctly, they should equate.

Some possibilities for error might be in the CT values and the meter multipliers applied. If these are validated to be correct, remember to apply the multipliers for each meter to the reading before adding them.

Since there will be differences in the time of the reads, unless you have 8 people reading at the same moment, and since there is a multiplier involved for the reads, the addition may be off by the value of each multiplier, but will balance out when several months are added together.

You gave no value to the difference in the reads, so we have no way to know how much difference you are working with. You hold the answers to all your questions by having the reads to work with. Does the main meter equal the 6 meters without the PF correction meter? Does the main meter show more usage or less usage? Are the difference values greater than the addition of the meter multipliers? (the read can be off by 1 digit X the multiplier, for each meter, depending on the timing of the reads.)

As for the capacitor bank for PF correction, yes there is some power expended in that process, mostly due to I²r losses. That branch should have metering that provides a net result, inflow - outflow, with a fairly small net usage compared to the other branches.

If the metering is done correctly, the values should fairly closely match the main.

Regards, CJM

Dear CJM,

Thanks for your helpful response.

The sub-meters only add-up to 62% of the main one. My first task will be to check all CT ratios today, and see if the CT's are installed correctly.

What do you mean by the multipliers? Do you mean KWhr vs MWhr?

It is true that the PFC meter reading is quiet low, 10,856KWhr out of 2,412,086KWhr for the whole board over 12 months.

In another switchroom with a very similar set-up the sub-meters add up to 102% of the main meter. So I suspect there is something wrong with the set-up.

Martin.

Martin, thanks for the further detail. I would invite you to sign up and log in to CR4, which helps anyone who may want to contribute to your question with details such as your region on this big globe. That often helps us understand what system differences may exist etc.

As far as your comment "What do you mean by the multipliers? Do you mean KWhr vs MWhr?" I find this very revealing and may lead to your discovery of the read addition differences.

With a service of your capacity, I knew it could not be directly metered and would require Current Transformers and depending on the voltage possibly voltage transformers in order to accomplish the metering. Whenever these are used in the metering loop, the reads of the meter will need to be multiplied by the ratios of the transformers to achieve accurate results.

The meter can only meter what is given to it, and if that is only a fraction of the actual power, a multiplier must be applied to correct that value to match the actual power.

Depending on the design of each meter, there may be a multiplier required for the meter itself, usually stated on the faceplate of the meter. Most modern meters have been designed to have a direct read with a meter multiplier of 1.

CT and PT ratios equate to a numerical value. For instance, 2000:5 CT's would reduce mathematically to 400 (2000 divided by 5). A meter would then receive only 1/400th of the actual current. The resulting meter measurement would then need to be multiplied by 400 to reveal actual usage.The same for a PT reducing 12kv to 120 volts; the ratio 12000:120 = 100.

Therefore the part of the meter multiplier resulting from the CT's and PT's would be (400 X 100) 40,000. If the meter factor is 1, then all reads would have a multiplier of 400 X 100 X 1 = 40,000.

A sub-meter probably has no PT's and the CT values will be much less, resulting in multipliers of 20, 40, 200 etc.

Additionally, if the main meter was read after the sub-meters were read, and enough time had elapsed to advance the main meter by 1 digit, the reads would differ by 40,000 kWhr. Over time the differences would equalize, since nothing was actually wrong, and 40,000 kWhr is nothing compared to 5,000,000 kWhr for the year for example.

SUMMARY

• Validate correct meter wiring
  
• Validate meter multipliers for each meter
• Read meters very close together in time
• Read main before and after sub-meters
  
• Allow differences equal to 1 digit X multiplier for each meter
  

Regards & happy hunting. CJM

Hi,

Have you considered automatically collecting the data and automating the reporting with network or wireless enabled power meters? Not as expensive as you might think and with a very rapid ROI !

Contact us for more information.

Chris J Knight

ANEC Limited

+44 1283 222121

anecltd@gmail.com