3 phase 440ac fuse+switch isolation

Thread Starter

spkr cabinet

Joined Jun 17, 2008
My question is about isolation of a 3 phase 440ac fuse+switch box.
Here is my scenario:

3 phase 440ac is fed from a main breaker to this fuse+switch box(i'm not sure this is the proper name of the box).

I needed to disconnect the normal load from this fuse+switch box and connect another load.

I turned off the main breaker that supplies this box.
Measured every connection point there is inside this box(both across all phases and phases to ground)with my dmm on Vac. All measurements were below 1V.

So i assume that this box is safe to work inside(to remove normal load connection and make my connection for another load).

I go to remove the screw holding the stakon on from the normal load and ZZZZZap.

After getting zapped I know that there must be some voltage on the screw that i tried to remove. So I put my dmm on Vac and measure everypoint again, <1Vac. I know that there must be voltage there because i felt it, I put my meter on Vdc and find that there was approx 100Vdc between each "load" terminal and ground.

I allowed these voltages to discharge slowly through my dmm and then it was safe for me to continue working.

Has anyone else had similar experiences to this or have any info that they would like to share on this situation?
Thank you for your time in reading my post. Any info you can pass on to me and others is greatly appreciated.

Thread Starter

spkr cabinet

Joined Jun 17, 2008
I later found out that this was feeding a high voltage power supply. Hope this thread saves somebody else from doing the exact same thing. Check for both ac and dc in a breaker panel/fuse+switch box after its supply is turned off.
Last edited:


Joined Apr 20, 2004
The situation is common enough that Navy ET's bridge bus bars with a chunk of wire (insulated where they hold it) to look for residual voltage. You never know.