Safety from shock can sometimes be ensured by separating a system completely from others so that there is no complete circuit through which shock current could flow. It follows that the circuit must be small to ensure that earth impedance's are very high and do not offer a path for shock current The source of supply for such a circuit could be a battery or a generating set, but is far more likely to be an isolating transformer with a secondary winding providing no more than 500 V. Such a transformer must comply with BS EN 60742, having a screen between its windings and a secondary winding which has no connection to earth.
There must be no connection to earth and precautions must he taken to ensure, as far as possible, that earth faults will not occur. Such precautions would include the use of flexible cords without metallic sheaths, using double insulation, making sure that flexible cords are visible throughout their length of run, and so on. Perhaps the most common example of a separated circuit is the bathroom transformer unit feeding an electric shaver. By breaking the link to the earthed supply system using the double wound transformer, there is no path to earth for shock current
The difference is that if more than one item of equipment was used and both had faults to say earthed pipework ( live of one and neutral of the other)you would have a shock potential
This cant happen with one item
Selv is extra low voltage,so you can have many faults ,but it cant give you a shock potential
