| Electronics > Beginners |
| Question: Isolation transformer, Ground floating or Ground shorted to Neutral |
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| SparkyFX:
The discussion around connecting ground and neutral strongly depends on the question if we are talking about an IT/TN/TT grid. The ops question was about an IT grid. Connecting the chassis of a device to protective earth only protects you as long as - the voltage source is grounded as well (should a mains wire have a short to the chassis, it trips the breaker by high current, therefore shuts off mains) - the person is grounded and only connected to one lead at a time The purpose of the isolation transformer is to protect by transforming voltage potential WRT ground to voltage potention not related to ground, thus not have the (carefully isolated) secondary side connected to ground. Therefore a grounded person will not have to suffer potentially dangerous current flowing through to earth should some part with input voltage be touched. If earth and neutral on the secondary side are connected all of this protection would be gone - there would be no benefit of using an isolation transformer. Use it to be able to inspect potentially defective devices with a faulty live chassis. There are of course special cases, e.g. as you put the DUT on the bench it could touch something that is grounded and through a ground fault short the setup becomes dangerous, so you got to know what you are doing at all times. --- Quote ---Ground shorted to neutral, NEVER! --- End quote --- Before the (properly installed) GFCI it is quite common to have Neutral and Ground connected near the house distribution panel (in TN-C-S grids), it will however trip the GFCI all the time if it was connected behind the GFCI, as the return current is split up between neutral and ground and exceeds the GFCIs value. It is not common in TT-grids. It might also lead to live chassis all over the place should neutral and ground be interrupted one day (which is more likely if they were connected to save a wire). So never mixing up both is a good practice. |
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