Instrument Transformer(s)

[I.T]

Hi everyone.I'm reading article 250,Part IX , Direct Current Systems of 2017 NEC. My question is that : what mean by Instrument Transformer(s) ?

 

[paulengr]

Hi everyone.I'm reading article 250,Part IX , Direct Current Systems of 2017 NEC. My question is that : what mean by Instrument Transformer(s) ?

Exactly what it says...PTs and CTs on AC circuits. You can sometimes use a little power off a PT but it will be fairly limited, say 50 VA.

Since this is for DC you can’t have an AC transformer. You can but there is no induction. So the current measurements are done with a shunt with ratings like say 1 mV per A (a 0.001 ohm resistor). Voltage is done with a resistive voltage divider, not a transformer. The exception would be a pulse transformer.

 

[Carultch]

Hi everyone.I'm reading article 250,Part IX , Direct Current Systems of 2017 NEC. My question is that : what mean by Instrument Transformer(s) ?

Instrument transformer is the general term for current transformers (CT's) and potential transformers (PT's). Alternative name for potential transformer = voltage transformer (VT).

It means a device that proportionally scales down the voltage and current of the circuit you are measuring, so that it is at manageable levels for the circuitry of the metering equipment to accurately measure. The scaling ratio is then programmed in to the device, to translate the measured voltage or current into a corresponding value on the main circuit. With AC, it is a small VA-rating transformer that senses a sample of the circuit, and rescales the mix of voltage and current in proportion to the turns ratio.

In DC, it isn't a transformer by strict definition, because transformers only work with time-varying voltage and current. Transducer is a more general term that would apply instead. A DC potential transducer would likely be a voltage divider, and a DC current transducer might use the Hall effect or induction sensing technology. It may seem just like a current transformer from the outside, but has more detailed inner-workings to make it compatible for DC current measurement.

 

[I.T]

Thanks guys.