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star A.C. Devices
star A.C. EMI Filter
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star Isolation Transformer - Safer Power Supply ?
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Isolation Transformer Safety

Is it really safer than the normal power supply from the socket on the wall ?

Search the Internet, you may find lots of information telling you that Isolation Transformer provide you a safer power supply. It is because both output lines are not connected to the earth as the normal A.C. does. Unless you touch both output lines of the Isolation Transformer, you will not be shocked. Is it true ... ...

Theoretically, it is true. But, there are many accidents ...

For the following diagram, the assumption is that the resistance of our body is much much greater than that of every thing. In order words, electricity would like to go through other paths rather than our body.

If the humidity is high, our body is wet, the resistance of our body decreases and safety factor will be reduced a lot, and we may get shocked even all protective devices are functioning.

This shows the normal A.C. you can find at the electric socket on the wall.
If the Live wire is accidentally shorted into the earthed case of the appliance, the current will go through the path in red. Therefore, we are safe.
This time, the case is not earthed correctly and the current will go through another path which is our body. Therefore, we get shocked.
Electricity Safety

If the RCBO / RCD is installed as shown, although the current will go through our body, the RCBO / RCD will trip when the difference of I1 and I2 is 30mA (0.03A) and save our life.

For more information of  RCBO / RCD, please refer to RCBO / RCD

The diagram shows the connection of Isolation Transformer. You can see there is no earthing at the supply (No Neutral line)
The diagram shows that there is no complete circuit even the Live line is shorted into the case accidentally. Therefore, we are still safe and most people think that Isolation Transformer provide a safer power supply.
Now, I am going to consider more situations ... and find out : Is it really safer than the normal power supply from the socket on the wall ?
Safe

Safe ??? Even both output lines are shorted into the case, normally, it is safe.

But, do not forget the assumption at the beginning and pay attention ... you only have one small small small point touching the case. If not, for example, you use two finger to touch the case, the current may enter your body through finger 1 and exit through finger 2. You become a parallel resistor.

In this case, the Isolation Transformer will be short-circuit and give out lots of heat.

If MCB / RCBO is installed between the Isolation Transformer output and the appliance, the MCB / RCBO trips by overload, the coil of the Isolation Transformer is saved, but our life may not be saved because at least 6A current pass through your body before RCBO / MCB trips.

If only RCD is installed ... ... useless.

Safe ... in this case, both output lines are earthed because the case is earthed.

Therefore, the body have the same electrical potential as the case and the output lines.

In this case, the Isolation Transformer will be short-circuit.

MCB or Fuse can save the coil.

To this moment, Isolation Transformer provide a safer power supply. We will not get shocked as long as we do not touch both output line at the same time.

But, in our daily life, we will plug more than one appliance into the same socket.


Safe

Safe

Safe, normally, both cases have the same electrical potential.

Safe ??? depends on the resistance of our body compare with the resistance of the appliances. 
The red line shows the probability of getting shocked.

Safe, both cases are earthed.

Only one case is earthed probably. Moreover, both appliances have electrical leakage ... ... this is a rare case; however, you may get shocked because one of the output lines is earthed through the case.

The whole system becomes a normal A.C. supply which one of the output lines is earthed.

For this case, RCD cannot save you.

If RCBO / MCB is used, RCBO / MCB will trip by overload; however, at least 6A current pass through your body before RCBO / MCB trips.

Therefore, it is unsafe !!! 


Touching only one output line of the Isolation Transformer is safe ?!

This diagram shows you are shocked. This case is similar to the case above. The whole system becomes a normal A.C. supply because one of the output line is earthed through the case.

For this case, RCD cannot save you.


Safe ... Although one of the output line is earthed through the case and becomes a normal A.C. supply system, we touch at the earthed output line ( Neutral Line ).

I am not sure I considered all situations; however, earthed appliance is always the safest for us even there is no MCB, RCD , RCBO, ...

Moreover, RCD may not save our life if Isolation Transformer is used.

According to the above situations, Isolation Transformer can provide a safer power supply when only one appliance is connected to it. For more appliances are connected to the power supply, Isolation Transformer may be more danger than normal A.C. supply.