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Stumped by CT question

Stumped by CT question

I was recently stumped by the following: consider a 800/400/1 Class P CT as per IEC 60044.1. The rated burden is given as 7.5VA (or 7.5ohms) at the 400/1 tap. At the 800/1 tap it is specified as 5P20. Is there a way of determing the rated burden on the 800/1 tap based on the information provided above?


RE: Stumped by CT question

You don't have a photo of the nameplate do you?
5P20 is 5% accuracy at 20x rated current, rather than a burden.
Base assumption would indicate that the 7.5VA applies to both windings (even though it'd actually be different). (Possibly) Expensive option would be to get something like a CPC100 and check it, but not everyone has that option.

All the CTs I've seen to 60044.1 have only one burden on the nameplate, even with multiple taps.

RE: Stumped by CT question

Definitions from AS60044-1 which is a copy of the IEC Document. All relate to the secondary output, so your rated burden is 7.5VA regardless of tap.

the impedance of the secondary circuit in ohms and power-factor
The burden is usually expressed as the apparent power in voltamperes absorbed at a
specified power-factor and at the rated secondary current.
rated burden
the value of the burden on which the accuracy requirements of this specification are based
rated output
the value of the apparent power (in voltamperes at a specified power-factor) which the transformer
is intended to supply to the secondary circuit at the rated secondary current and with
rated burden connected to it


RE: Stumped by CT question

I think a distinction must be made between burden and rated burden. If I read the definitions correctly burden refers to internal (RCT) as well as external (RB) burden.

Rated burden refers to the external burden of the CT when rated current is flowing, etc. Thus by definition, if the rated burden is 7.5ohms at the 400/1 tap then it cannot possibly be 7.5ohms at the 800/1 tap.

This can be explained by supposing the internal burden is RCT and the external is RB. Consider the 400/1 tap. Suppose rated burden is connected. The maximum output the CT can deliver whilst still obeying its accuracy class is when ALF current is flowing.

The VAmax = ALF2*RCT + ALF2*RB(1A CT). Now VAmax is the same whether on the 400/1 or 800/1 tap. With the 800/1 tap RCT is double that of the 400/1 tap which means RB must be less to get the same VAmax.

What stumps me is that without knowing what RCT is (it is not part of the Class P spec) it is not possible to know by how much RB must be reduced. Or is there a way?

RE: Stumped by CT question

I must admit I've never had a multiple tap 60044.1 CT tested to see, but all the AS1675 CTs have always had a knee point voltage above the nameplate voltage specified.

What I would expect is that the nameplate burden is the worst case burden to be connected to it, and is tested for the worst tap (Say, 800/1 as per your example). This would mean that the available burden on the 400/1 tap should be well above the stated 7.5VA, but again, that's an assumption on my part. If I get a chance I'll have another look at the standard, certainly there are testing restrictions, and the issue of not knowing what the internal resistance of the CT is; is a well known one, if the Cahier's Techniques from Schneider Electric are anything to go by.

RE: Stumped by CT question

Yes, the AS1675 CT's specify a secondary reference voltage which in essence is the maximum CT terminal voltage with the CT still being within the declared error. Induced emf is then the terminal voltage + volts drop across RCT.

Actually nameplate burden is the rated burden, but a higher burden can be accommodated if allowance is made for the fact that the ALF will have to come down.

Yes, I have a perculiar situation where I am told the CT is 7.5VA at 400/1 tap but 5P20 at 800/1. Why on earth could the manufacturer not provide all the specs at one tap. Sorry, I have no nameplate details but am chasign the manufacturer.


RE: Stumped by CT question

Starting to sound like you ended up with the graduate providing the CT details for you...

RE: Stumped by CT question

Per IEC, if there is only 1 burden and protection class given it is applicable at the lowest tap. From a practical perspective, you can assume the burden rating would double, as the ratio doubles, while keeping the protection class rating the same. For example,

If you know 5P20 - 7.5VA @ 400:1

you can safely assume 5P20 - 15VA @ 800:1

It's conceivable (and probable) that the nameplate is listed as 400/800:1A, 5P20-7.5VA, e.g. not a separate rating given for each tap.

Note that this is only valid for tapped secondary units and not primary re-connectable (series/parallel primary) units.

RE: Stumped by CT question


If you know 5P20 - 7.5VA @ 400:1

you can safely assume 5P20 - 15VA @ 800:1

I think it is the other way around – i.e. reduced burden (or ALF) at 800/1 tap. Consider the 400/1 tap with rated burden connected. With ALF current flowing the VA delivered by the CT is the maximum output the CT can deliver and is given by 202*RCT+202*7.5 = 400*RCT + 3000. Now this value is constant as the CT cannot deliver anymore VA without going into saturation (or error becomes > 5%).

This output, call it VAmax, is split between the internal VA requirement and the external VA requirement, i.e. VA max = VA(RCT) + VA(RB). Now if RCT doubles then RB or the ALF must come down as VAmax is a constant.

I hope my explanation makes sense. Note that I cannot know by how much RB must be decreased as I do not know the value of RCT. According to my understanding in any case.

This leaves me with the next question:
If a CT is given as 5P20, 7.5VA at 400/1, what is it's spec at 800/1? Again I cannot see how the CT can be spec’d at a different tap unless RCT is known.

I have put the question to the CT manufacturer and am eager to see what they come up with.

RE: Stumped by CT question


You are neglecting the fact that the 800:1 ratio will have twice the turns around the core, meaning that the flux density will go down by approx. half. Therefore, it can generate roughly twice the terminal voltage, hence for the same ALF, it can be rated with twice the burden. In other words, you're not considering the "magnetics" change with the different ratio.

For Rct, it is safe to assume that the same size secondary wire is used for the entire winding, therefore Rct @ 800:1A is approx. twice the Rct at 400:1A...once again, assuming this is a tapped secondary and not a primary reconnectable design.

I work for an instrument transformer company and it is quite standard to have the burden rated twice as high at the full winding of a center-tap secondary.

RE: Stumped by CT question

Will think about this. I agree with your middle paragraph but the first one? At maximum flux for a 5P is around 1.7Teslas. Yes, the more turns the more the induced voltage, I agree. But by the conservation of energy the CT can only transfer X VA to the secondary winding, irrespective of the turns and secondary load.

Will give it some more thought.

RE: Stumped by CT question


I think you're mixing parameters a bit.

7.5VA burden = 7.5 ohms burden (7.5ohm * 1^2)

5P20-7.5VA @ 400:1A tap is 150V across the secondary (20 * 7.5ohms * 1A)

5P20-15VA @ 800:1A is 300V across the secondary (20 * 15ohms * 1A)

Think about an excitation curve plot showing multiple ratios for the same unit/core. The higher ratio has a higher knee-point voltage. The Vkp change is approx. proportional to the ratio change. So double the turns means approx. double the Vkp, which equates to double the burden, assuming the ALF remains the same.

So basically a unit rated 5P20 - 7.5VA could also meet 5P40 - 3.75VA, since 20 * 7.5 = 40 * 3.75.

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