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Anonymous Poster

Why doesn't the transformer gives me a shock?

03/17/2008 2:07 PM

Suppose for magnetic balance test I apply 230V rms between R-phase and Neutral of HV side of YNd1,40MVA,66/11kV transformer from a Distribution board with only MCB(miniature circuit breaker) as the protection and suddenly break this(switch off supply) circuit by swithching off the MCB why i am not experiencing any shock due to inductance of the transformer winding.Inductance has got electrical inertia and it will never allow the current to change suddenly.So why an overvoltage is not produced in this case??

CONSIDER the priniciple of how a normal fluorescent lamp/tubelight glows

In normal fluorescent light/tubelight the ballast(inductance) does the role of initiating the arc in the tube light.when we apply 230V this voltage comes across the starter of the fluorescent tubelight and a small arc is produced across the starter.due to arcing the circuit is complete but as soon the bimetallic starter contacts cools down the circuit is broken.Now the ballast connected in series to the supply will oppose the change in current and generates a voltage of around 800V in series to the supply voltage say 230V and this 1030V(approx) will initiate the arc in the fluorescent lamp and hene the flourescent lamp glows?

My question is that when an ordinary ballast boosts up the 230V to around 1kV why dosent the transformer winding with a high number of turns does the same?

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#1

Re: Why doesn't the transformer gives me a shock?

03/18/2008 12:22 AM

What makes you think it is not?

You need to think that through a little more. just because it is a HV transformer does not mean it has a high turns ratio. Look at the specs you mentioned. 66kV to 11kV is only a 6:1 ratio. So if there is any inductive kick when you open the 230V side, you would be hard pressed to see it without a very sensitive scope.

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Re: Why doesn't the transformer gives me a shock?

05/03/2008 8:25 PM

You will get an inductive kick back but the transformer characteristics can mask it. This very large transformer has a lot of distributed capacitance that will filter out the voltage spike from the interrupted current. You must also consider that the magnetizing inductance for such a high voltage transformer is very large. This means that very little AC current will circulate in it at 220V. The energy stored will be very small. Therefore the inductive kick will also be small. If you would try the same with DC current, you could get very dangerous inductive kick back even with as little as a 12VDC car battery. Be careful, you could injure somebody or destroy something.

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