A Class exporting DC volts!

I have a pulsacoil A class installed with both a 24hr supply & separate off peak supply via a split consumer unit, with a mcb for the 24hr & mcb for off peak. There is also a wall isolating switch with LED for both 24hr & off peak supplies.
I noticed that when the off peak supply was off (not live) the LED on the wall isolating switch was still lit, although not quite as bright as the 24hr LED.
An electrician has checked & the unit is wired correctly for the two supplies. However what they did find was that with the 24hr supply on, the pulsacoil unit exports a -89vDC back to the mains on the off peak supply. This is what is lighting the LED. My question is, should the pulsacoil be doing this (i suspect not) & what is causing it??

Any advice would be welcome.