Multiple relays sharing CTs consideration

GoldDigger

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You first have to put the relay coils in series, not parallel.
Then you have to add up the total series resistance along with wiring resistance and make sure that it is lower than the rated "burden" of the CT.
 

mbrooke

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You first have to put the relay coils in series, not parallel.
Then you have to add up the total series resistance along with wiring resistance and make sure that it is lower than the rated "burden" of the CT.
Ok, that makes sense now that I think about it. However, dont I need shorting switches for each relay?
 
Also consider that a fault on the CT circuit will affect all relays on that circuit. Always best to put primary and backup relays on separate circuits. Also applies to adjoining zones of protection.

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GoldDigger

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Staff member
I would but it needs to be so the relay can be taken out of service will keeping the other on line. Any idea how much impedance one relay holds? Or where I can find it?
It should be in the relay specs. Look for rated burden in ohms. If not, call tech support.

P.S. I don't understand why you need to attach multiple relays to one set of CTs.
Different thresholds?
 
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mbrooke

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It should be in the relay specs. Look for rated burden in ohms. If not, call tech support.
will check, thanks :) Once I know the value, how do I factor the rated burden into the programming? Simple equation will do?


P.S. I don't understand why you need to attach multiple relays to one set of CTs.
Different thresholds?

Primary and back up relaying (redundant protection). Main and transfer application also call for it as the buss coupler CT is strung rack to rack.
 

Ingenieur

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You first have to put the relay coils in series, not parallel.
Then you have to add up the total series resistance along with wiring resistance and make sure that it is lower than the rated "burden" of the CT.
this ^
the CT will have an allowable burden (with cabling, etc.)
the relay(s) will have a load burden

burden is usually rated in VA though?
 

Ingenieur

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it seems some use both std 5 A ^ 2 x |Z| = VA
I could see where Z would have an advantage though
Total Z (wire, ct, etc) x std 5 A ^2 = total VA load
 

mbrooke

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GoldDigger

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Im confused :?
A current transformer's output is a current which is proportional to the input current as long as the load impedance is low enough.
It is intended to go into a current measuring device, which must be low impedance.

If you want to get an output voltage which is proportional to the input current, you can use a Hall Effect device or connect an op amp and shunt to the current source.

One reason to use a shorting switch at a CT is that an open-circuited CT will produce a messy voltage waveform which is determined largely by the saturation characteristics of the core and it can be dangerously high if the core does not saturate easily.
 

mbrooke

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A current transformer's output is a current which is proportional to the input current as long as the load impedance is low enough.
It is intended to go into a current measuring device, which must be low impedance.

If you want to get an output voltage which is proportional to the input current, you can use a Hall Effect device or connect an op amp and shunt to the current source.


But say you just want it to trip the relay on a certain voltage? A simple resistor (sized correctly of course) will do?
 

GoldDigger

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But say you just want it to trip the relay on a certain voltage? A simple resistor (sized correctly of course) will do?
Yes, as long as the resistor is smaller than the rated burden of the CT. But the output voltage will be very low in that case.
Then you could put multiple high impedance meters in parallel (and do not short any of the sockets!)

But if the relay you choose is designed to connect directly to a CT it will implement a current input.

PS: There is no way a CT can tell you the voltage on the wire it is connected to. For that you need a PT physically connected to two reference points.
 
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