the voltage between neutral and ground is 1.5 v but when i swithon on panel air conditioning the voltage between neutral and ground becomes 6v .pl advice how to correct. the panel air conditioner is 230 v ac single phase

Since the conductors we use are not exactly 0 Ohms this is normal. As the current in them goes up so does the voltage you measure on it. The only thing you can do to compensate for this is to go to a larger size conductor. The longer the wires are the higher the voltage you will measure. So when you turn on the A/C the current in the wire goes up which leads to a voltage increase as you measure it.