Does anyone have a good circuit schematic of a current transformer, and how it is connected to a source and load. I've been reading about how opening the secondary leads of a CT can introduce a dangerous voltage, however I am having a hard time visualizing it. I was hoping maybe a good circuit schematic would help.
Current Transformer Circuit Schematic
- Thread starter mull982
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- Wisconsin
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mull982 said:
Remember how a transformer works. The current and voltage move in opposite directions. Effectively, when you step down the current the voltage goes up by the same ratio.
LarryFine
Master Electrician Electric Contractor Richmond VA
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- Henrico County, VA
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- Electrical Contractor
How about a description. A CT allows a large current to be read indirectly using a representational current, so the meter need not carry the actual current. The meter's readout must be matched to the CT for accurate measuring.mull982 said:
A typical CT with, say, a 400:5 ratio, which is the same as 80:1, has 80 turns in the secondary, so, for each amp in the single conductor passing through the 'donut", there is 1/80 of an amp through the meter, or 1 amp in the meter for each 80 load amps.
Note that the CT's secondary circuit is of almost zero impedance, as most ammeters are, so little voltage is developed; it's all current. However, if this circuit is opened, the CT now functions as a PT (potential transformer), with a step-up ratio of 1:80.
An extremely high voltage can be developed with just a small amount of primary current. Usually, the first damage is to the insulation of the secondary winding from arcing, but external arcing can also occur, along with personnel hazards.
LarryFine said:
Thanks for the explanation!
I was trying to visualize though why when you opened the secondary of a CT you could have a high voltage, however doing the same with a PT you will not. Are these circuits arranged differently? I was reading something that was explaining that is had to do with the fact that a PT's primary was in parallel with the source and load, where the CT's primary was in series with the source and load. This description was the part I was having trouble visualizing.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
You're welcome. A few more tidbits:
A PT is a step-down transformer, while a (non-shorted) CT is a step-up transformer. A standard watt-hour meter measures both source voltage and load current; remember that watts = volts x amps.
A CT setup allows a meter to measure actual current by using a representational current. A PT/CT setup does the same thing with current and voltage. Even a straight CT setup also requires a tap on each line for voltage.
Picture a high-current meter that reads out actual current by measuring the voltage drop across a shunt resistor. The meter is a volt-meter, but the dial face is printed to read out the current. They must be matched.
A PT is a step-down transformer, while a (non-shorted) CT is a step-up transformer. A standard watt-hour meter measures both source voltage and load current; remember that watts = volts x amps.
A CT setup allows a meter to measure actual current by using a representational current. A PT/CT setup does the same thing with current and voltage. Even a straight CT setup also requires a tap on each line for voltage.
Picture a high-current meter that reads out actual current by measuring the voltage drop across a shunt resistor. The meter is a volt-meter, but the dial face is printed to read out the current. They must be matched.
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