CT Ratio

The CT is for measuring the out of balance current (and possibly tripping a circuit breaker). Your ratio will be what ever you need to feed into your control gear, for example a 400:5 ratio CT will give you a 5A output when the primary current is 400A.

The 400 A means that 'box' termed a neutral earthing resistor will limit the current on neutral to 400 Amp, a typical USA standard for neutral fault current limit. Now 400 Amps is still pretty hefty so you also need to trip a breaker or something to interrupt the 400 Amps flowing through your neutral so they use a CT (Current Transformer) to sense current flow, limited to 400 Amp, and connect the secondary wires of the CT to that tripping device whatever it may be. So look on the drawings for a CT, it is shown as a coil with the neutral lead passing through it and a black dot on the drawing indicating direction of current flow. There will be a ratio shown on the drawing near the coil such as 400:5 as our other friend pointed out (400 amps going through the donut coil = 5 amps on the 2 wires coming out of the CT. The CT is directional sensitive so be sure the dot on the drawing matches the dot on the CT (current flows into the dot) and also the dot on the CT may be more near one of the two small wires and this is the high side wire. Sometimes it is a black wire. If this is all confusing get a professional to help out as CTs can kill you. If there is current flowing through the CT donut hole, and the secondary wires are open (not shorted together or not connected to a resistive load like a device) the CT will build millions of volts in a few seconds and literally explode in your face. Careful here !!!!

Thanks for the comment. On the one line drawing that our customer gave us, they show the NGR with CT's on either side of it. To the left is (3) CT's going to something called "Volt Restrained PH Overcurrent" and to the right of the NGR is (1) CT going to "Frequency". On all the other CT's in their drawings, however, they do show a ratio. For Ground Fault Window Type CT's they show a 50/5A ratio, and for normal CT's going to "Overload" Coil they have100/5A. The NGR vendor is asking me what I want my ratio to be, and no one here seems to know how to calculate that (400:5A?) Are these other CT's for metering? Thanks for the help.

The information you list is off the NGR, and is to limit fault current. It will handle 400A, at 4160 volts for 10 seconds. The secondary side of the 400:5 CT is connected to a protective relay that is in the trip circuit to open the main breaker to limit the fault current before it blows the place up.

Welp, I'd hire a EE to sort this out. I don't understand all these terms such as a "CT going to Frequency". That one really confuses me. Sorry I can't continue to help.

You did not make your self clear.10 sec is not a transformer rating.Do you want 2 know the necessity of CT?It's a protecting device use to step down the line current to a minimum value.From your information the turn ratio are not given.In my country(India) 200:5 is a common practice.

The CT on the neutral grounding resistor is for the ground fault or earth fault protection. In the event of a phase to ground fault, the current will return through the resistor to the neutral. The resistor will limit the magnitude of the fault current and the CT will send an output to the earth fault relay for operation.

The ratio of the CT is normally 50/5, and this is indicated in the drawing.