Why an Open Secondary Circuit CT is Dangerous???

I think you're asking about a 1:400 step-up transformer? If you have an open (or even lightly loaded) secondary, when you open the primary, you get a big voltage across that secondary. You have only to look at the coil and points on an older car.

Friend a CT basically steps down current at secondary side i.e it steps up voltage at the secondary.So it becomes a step up transformer as transformers are termed step-up or step-down if they are stepping up or stepping down voltage respectively.

I am talking about a 400/1A CT( the primary full load current = 400A & secondary current = 1 A) which is mounted on the bus-bars say of GIS in a substation.

<...open secondary CT....may become dangerous as high secondary voltages are produced...>

Correct. For information, this is why circuits fed from a current transformer are not fitted with fuses.

Oh, CT = current ransformer. I thought Center Tap. Duh. I may need to visit my neuron (I keep it in a jar of single-malt at the National Aviary) more often than just on senior discount days.

The mistake in this argument is

Ip is only dependent upon Ep .i.e. Ep is not equal to the product of Ip x Rp .Even when there is no load on the primary,the EP will be there .

Yes, I've just talked to someone cleverer than me (there are lots of those) and the last post didn't describe the process correctly. Transformer action couples impedance by matching current actions according to the turns ratio. Therefore as the current stops in the secondary due to infinite impedance that impedance is coupled back to the primary where it attempts to stop the current flowing, this will cause (most of) the applied (mains) voltage to appear across the primary which in turn induces a pro-rata step-up voltage in the secondary.

This implies 240V (say) x 400 = 96000 V in theory causing insulation breakdown and fire in the secondary. I hope this helps.

Good explanation.

It is definitely true, we've had to rescue the same bloke from CT terminals twice.

I've seen where open circuited secondaries arced their way around and eventually through terminal insulation.

I've seen where intermittent open circuits in even small CT secondary leads have blown up electronic circuits in a big way.

Remember it is all about Ampere turns, when the turns can't push the Amperes through the burden resistor due to high resistance or open circuit the voltage must rise.

Not to get too far off topic, but for those who don't understand there is a more real danger of the current in an open secondary on a CT. Many CT's for industry are rated at 2.5VA or higher, which is more than enough current to stop the old ticker if you suddenly become the load for an opened secondary. For this reason, we put warning labels on all of our CT's. The voltage is almost a secondary danger in comparison of imminent lethality.

It is true that the construction of a CT makes it look like a simple step up VT and the difference is in the usage. In a CT the primary voltage drop is determined by the secondary voltage drop into the load. In transformer applications the secondary load is reflected to the primary as Zpri = Zsec * (Np/Ns)^2. So, a 50 ohm terminated 1 : 400 CT loads the primary coil with 0.313 milliohm. Higher output impedance (your fingers for example) will present a higher impedance on the primary allowing for higher voltage drops on both sides. The primary impedance will be limited by the transformer coupling and the primary inductance regardless of the secondary load impedance. In short you can get a pretty high voltage at the secondary.

Voltage & current are relative to each other (Ohms law). If you have a CT that is a 400/1 ratio 400A on the primary side will only produce 1A on the secondary side. If your current is devided by 400 then your voltage must be multiplied by 400, hence giving you a dangerously high voltage if no burden is conected

Ohm's law E=IR. Open circuit on secondary can be thought of as infinite impedance, R, thus E rises in proportion.

Dear Mr junaid,

The voltage is multiplied by the Ratio of C.T. In your case Primary Current is 400 Amps. and Secondary Current is 1 Amp. Hence Ratio 400/1 = 400, and there fore Voltage on primary side will be multiplied by 400 times, and hence open circuit on secondary is DANGEROUS.

DHAYANANDHAN.S