CT for digital relays

[voyager2]

I understand that some years ago, when there weren't digital relays, a CT for measurement and another CT for protection were used, and now, after digital relays come into market this concept was changed. I mean, in this last case there is just one CT for measurement and protection (the same CT). Is it that true ? Is there a NEC norm (or another one) for this ? Which one ?

Best regards,

Voyager2

 

[rcwilson]

Re: CT for digital relays

IMO ? Technically one CT can usually do the job, especially with the low burdens of digital relays and meters. I am not aware of any "standard" or recommended practice. It's a matter of philosophy.

Protection CT's need to work on high fault currents, a few times the CT rating. Metering CT's need to be accurate at currents from 1-100% of the rating. With electro-mechanical relays and meters and their higher burdens, sometimes different CT ratios for metering and protection were required to guarantee proper operation.

Some owners/utilities do not want metering and protection on the same circuit. Metering usually has test plugs or other means for testing the meter. Metering gets looked at and fooled with more than relays. Keeping the circuits separate minimizes problems like shorting out a relay during testing.

If the metering is only needed for operation and control and not billing purposes, many digital relays can provide the required metering signals.

But, we have had installations where being able to compare the two CT currents on the same circuit provided useful information.

 

[petersonra]

Re: CT for digital relays

I think this is also somewhat of a design philosophy issue.

many engineers feel you should have a seperate unit for trips versus display and controls, and not just for switchgear, but for instrumentation in general.

personally, I am inclined to believe two units are better than one, since you can use them to cross check each other.

for instance, I typically have a temperature switch paired with a thermometer, or a pressure gauge paired with a pressure transmitter. I want an independent means of quickly determining if a measuring device has failed.

the more critical the application is the more important it is to build in redundancy.