Magnum Inverter info from engineering

I have some feedback, but still haven't talked to Magnum Engineering yet...

Everyone,

Well, I’m making some progress, but still not satisfied that I completely understand what’s the reason that a Magnum Inverter can be damaged by disconnecting the negative battery terminal first. That’s because while I did receive an email from Magnum engineering relayed to me via the help desk, their engineers still haven’t contacted me directly to discuss the failure mechanism.

Failure Analysis

The theory is that unless you disconnect the positive battery connection on a Magnum Inverter first, it’s possible that a fault current can back-feed through the control wiring and burn up the inverter electronics. Of course, most of us (including RV technicians) have been taught to disconnect the negative battery terminal first before working on any DC electrical system since that’s the safest way to avoid shorting out a wrench or jewelry. But will doing that damage a Magnum Inverter?

Here’s my feedback email from Magnum Engineering

“Normally, disconnecting the positive or negative first shouldn’t matter.   There is an exception to this.  If the ME-BMK is installed or an older generation ME-AGS is installed, then the positive must be disconnected first.  The current AGS model doesn’t matter.
 
If the BMK is installed and the negative removed from the inverter first, the BMK will act as a Vbatt – (neg) return path.  Eventually this will damage the control board and the BMK.  This is why we suggest always disconnecting the Vbatt+ (pos) first.

Ideally, the user should be disconnecting the inline fuse or circuit break before removing the DC cables from the inverter.   

Regarding connecting DC to the inverter – our manuals are consistent I believe. Chassis ground is the first thing that is connected.  Then the over current protection (breaker or fuse) should be opened.  Then either positive or negative can be connected, in either order.  After positive and negative have been connected, then the fuse or breaker should be closed.  Properly insulated tools should always be used.”

What I don’t know yet

I’m trying to determine how disconnecting the negative battery terminal first on a Magnum Inverter would allow fault current through the BMK. For me to completely understand how damage can occur I need to see a schematic of the failure path and confirm my hypothesis with their engineers.

Be sure to disconnect the positive battery lead for Magnum Inverters first!

Until I learn something different and get confirmation from Magnum, my advise is to always disconnect positive DC power going to any Magnum inverter. Preferably you’ll have a battery disconnect switch on the positive battery terminal, but in any event you’ll want to use insulated tools.

I’m still waiting for Magnum engineering to contact me, but their help desk says I have to pay their engineers $150/hr to discuss it with them. I’ll keep you all updated as I learn more.

Let’s play safe out there - Mike