Current transformers

In distribution subs they have thermal ammeters I think they have a 30 minute warm up time so they don't get hammered by large starting currents or faults. I think I'm right in saying that the meters are switched out if circuit forr most of the time I'm you'd you can't leave CTs open circuited so do they just put a resistor across  the CTs in are the meters usually left in circuit.