Current Transformers
There are many different applications for and methods of using the Electromagnetic Industries products and we cannot be familiar with every particular system configuration that each of our products may be used in. For this reason we may not be able to give direct advice of how to set up your particular system. Any statements here are general statements that may or may not apply directly to your specific system configuration. We can only speak to how the Electromagnetic Industries components and products are designed to operate. Any questions concerning your complete system configuration or proper application of our products in your particular system must be addressed by your switchgear manufacturer and/or your EE.
Question: What does the "C" mean for Relay Accuracy Class CT's?
Answer: Relaying accuracy class designated by “C” describes the capability of the transformer. The “C” means the transformer ratio can be calculated, (considering the window type current transformer with uniformly distributed windings and a low-reactance design).
The secondary terminal voltage rating is the voltage which the transformer will deliver to a standard burden at 20x normal secondary current without exceeding 10% ratio error and the ratio error is limited to 10% at any current from 1 to 20 times rated current at any lower burden.
For example, relay accuracy class C100 means that the ratio can be calculated and that the ratio error will not exceed 10% at any current if the burden does not exceed 1.0 ohms (1 ohm x 5 amperes X 20 times normal current = 100 volts).
The secondary terminal voltage rating is the voltage which the transformer will deliver to a standard burden at 20x normal secondary current without exceeding 10% ratio error and the ratio error is limited to 10% at any current from 1 to 20 times rated current at any lower burden.
For example, relay accuracy class C100 means that the ratio can be calculated and that the ratio error will not exceed 10% at any current if the burden does not exceed 1.0 ohms (1 ohm x 5 amperes X 20 times normal current = 100 volts).
Question: Do you manufacture CTs with secondary currents other than 5A?
Answer: Yes, we can manufacture CTs with secondary’s other than 5A. The most common are 1A, .5A, 0.2A, 0.1A, 0.08A, but we can produce others.
Question: What is the polarity of the CT?
Answer: In most Instrument Transformer applications where the CT is being used for measurement of amperage magnitude only, the CT will work regardless of direction of current flow through the CT. However, typically the H1 faces source and H2 faces load. (refer to figure below)
In applications that look at Phase Angle, compare Phases or use Phase Vector Summation, the direction of current may be critical. In these cases, the CTs must all face the same direction.
All of Electromagnetic Industries CTs are marked with a minimum of H1 and X1. For units with black and white lead wires, the white wire is X1.
In applications that look at Phase Angle, compare Phases or use Phase Vector Summation, the direction of current may be critical. In these cases, the CTs must all face the same direction.
All of Electromagnetic Industries CTs are marked with a minimum of H1 and X1. For units with black and white lead wires, the white wire is X1.
Question: What is the Thermal Rating Factor (TRF)?
Answer: The TRF (Thermal Rating Factor) of a current transformer is the indication of the transformer’s overload capability. The number indicates the amount the primary load current may be increased over its rating without exceeding the allowable temperature rise.
In order to be completely meaningful, the ambient temperature the rating factor applies is stated. The standard ambient reference levels are at 30°C or 55°C.
In our typical sales literature, a statement may be: RF 2.0 at 30°C ambient with RF 1.5 at 55°C ambient. These statements mean that in a 30°C ambient, the CT will safely carry 2 times the nameplate rating and at 55°C ambient, it will carry 1.5 times the rating on a continuous basis.
In order to be completely meaningful, the ambient temperature the rating factor applies is stated. The standard ambient reference levels are at 30°C or 55°C.
In our typical sales literature, a statement may be: RF 2.0 at 30°C ambient with RF 1.5 at 55°C ambient. These statements mean that in a 30°C ambient, the CT will safely carry 2 times the nameplate rating and at 55°C ambient, it will carry 1.5 times the rating on a continuous basis.
Question: What taps do I connect to for the Multi Ratio units?
Answer: The typical Multi Ratio unit will have a schematic showing the number of turns between each tap. To determine the tap location to be used, divide the primary current with the secondary current. This will give you the number of turns you need for that given primary current. Use the schematic to determine the correct connection for your application.
Example: A typical 3000:5MR has 5 taps.
From T1 – T2= 200 Turns, T2 – T3 = 240 Turns, T3 – T4 = 60 Turns, T4 – T5 = 100 Turns.
T1 – T2 = 200 Turns (1000:5) T1 – T3 = 440 Turns (2200:5)
T1 – T4 = 500 Turns (2500:5) T1 – T5 = 600 Turns (3000:5)
T2 – T3 = 240 Turns (1200:5) T2 – T4 = 300 Turns (1500:5)
T2 – T5 = 400 Turns (2000:5) T3 – T4 = 60 Turns (300:5)
T3 – T5 = 160 Turns (800:5) T4 – T5 = 100 Turns (500:5)