Incoming Power

The 502 VAC coming in is less than 5% over the nominal 480 VAC, while the 139 VAC is over 15% higher than 120 VAC, and 21% higher than 115 VAC which is a closer nominal for 110-120.

However, if the nominal supply voltage should be 460 VAC, then you are about 9% over at 502 but this still does not account for the excessive 139 VAC.

It would appear that the problem lies in your step-down transformer. It may be defective, the wrong type, or is wired and/or tapped incorrectly.

The obvious answer is begin by consulting with your power provider, and then if necessary engage the services of a competent electrician to locate the exact cause of your overvoltage, hopefully before you have another fire.

Greg

thanks for the +_ 5% info as for the taps on my transformer when the incoming power is at 480 the tap im on is at 115 vac the tap below is 109 vac above 121 vac the trans is 40+ yrs old. just a foot note 4 days after the fire the 1920 vac mains to the elec co. transformer fried the power co pulled all new wire.