For the same voltage of AC and DC, the potential stress on the insulation is different in both cases.
The potential stress on the insulation is less in the case of DC system, than that in the case of AC system.
let say we have 220 Volt DC, it means the maximum value or in other words peak value of the voltage is 220V
but when we talk about AC with same voltage level of 220 Volt AC (As in our home distribution system = single phase AC Supply = 220 V AC or 110V AC in the US), then it is basically 220 RMS (Root Mean Square = √2) value, i.e. it is the RMS value of 220V AC.
The peak value of AC voltage is equal to √2xVRMS = 1.414 x VRMS (where √2 = 1.414)
In other words. The peak value of AC voltage = VRMS / 0.707
Now, in our case, The peak value of 220V AC = 220V/0.707 or 220V x 1.414 = 311V AC peak.
It means 220VDC = 311V AC peak. That’s why AC Voltage needs more insulation than DC voltages for the same rating of Voltage and Power.
In addition, 220 Volt AC can reach to its peak value of 220/0.707 = 311V peak. Its Mean 220 Volt AC RMS reaches to 311 Volts peak and again to -311V peak in one hertz (Hz) of frequency (where frequency = cycles/Sec).
That’s why AC needs more insulation than DC.