Current transformers

Question

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.

statter · Answer

The switch justs shorts the CT out. No need for a resistor. The current is defined by primary current and CT ratio.

gkenyon · Answer

Also worth considering that, without a resistor or short at the current transformer, the voltage will not be maintained in the SELV range. 
 You can buy current transformers that are guaranteed SELV, but they are very expensive. 
 For other current transformers, these need to be treated as FELV at least, if not LV, and preferably go in an enclosure. Similarly, insulated wires not in containment or sheath wouldn't meet BS 7671 requirements. 
 Only mentioning this, as current transformer installation is ramping up with PV inverters and EV charging equipment ... but not always installed according to BS 7671 requirements because the actual potential voltage range is not considered.

Current transformers

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