What is the advantage of using a multi-ratio CT vs a single ratio CT?

[bdn2004]

I'm looking at some new 12kV switchgear to shed load from the existing switchgears. All the existing have single ratio 600/5 CTs on their feeder circuits for protective relaying, but the new switchgear has 1200/5 multi-ratio CTs - set at 1200/5 from the factory.

It would be more intuitive imho if all the settings matched each other: old going to new. Is it worth telling them to change the ratio ?

 

[dpc]

Multi-ratio CTs make life easier during design - no need to try to size per expected load. In olden days, the CTs served both protective relays and analog ammeters. The maximum range of the ammeter was based on the CT ratio. So the CT needed to roughly match the expected load current. A 0-1500 A ammeter on a feeder with the maximum load of 100 A wouldn't be popular. These days, there are no ammeters, so not as big an issue.

But you don't get this convenience without a price. Tapping down a 1200/5 CT to 200/5 also reduces the relay accuracy class of the CT dramatically.

Also, there is a lower limit for relay pickup settings - and it isn't always all that low - especially for SEL relays.