Blondel’s Theorem

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Blondel’s Theorem is used to decide what type of metering installation is needed to meter a particular service. So, what does Blondel’s Theorem say and what does it do for us.

Blondel’s Theorem really is more simple than many people think. Blondel figured out what was needed to accurately meter different types of services. Blondel’s Theorem basically says that you need one less element in your watthour meter than the number of wires in the service to accurately meter the service.

That is all well and good, but what does it mean? Inside the watthour meters are stators, or elements. In this case we interchange the two. A stator inside the watthour meter is made up of one current coil and one voltage coil. So for example, to meter a single phase two-wire service accurately according to Blondel’s Theorem we would need a watthour meter with one stator because we have two wires. This meter is referred to as a 1s meter.

Let’s say that we now want to meter a three-phase, three-wire service typically referred to as a three-wire delta service. What would we need to use? According to Blondel’s Theorem we would need to use a form 5s meter which has two elements in it.

I think that now you have the idea. But I think it is actually easier to explain with CT’s. Let’s restate Blondel’s Theorem and instead of saying elements let’s say CT’s. So, to accurately meter a particular service we need to make sure that we have one less CT than the number of wires in the service. For example, let’s say that we have a three-phase, 4-wire wye service. How many CT’s will we need? If you came up with three then you are correct.

Now, I do want to post a disclaimer here and the reason is that there are non Blondel compliant meters and metering installations out there. The thing that we have to remember when we see these things is that Blondel’s Theorem is just that, a theorem and it is possible to meter services accurately without complying to Blondel’s Theorem. However, it is my opinion that it is much easier to attempt to use Blondel’s Theorem in your system and have uniformity across all of your installations than to say that you figured out a way that it would work without it.

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