Electrical issue on fire truck
 

Hello folks. I apologize if this is the wrong place to be psoting this message, but I can't find a message board related to fire trucks and I have to believe there are some truckers out there with some similar experience to the scenario and system below.

Keep in mind I am trying to solve this problem for a friend over the phone. The truck itself is over 200 miles from me so I may not have all the information needed. here is what I know...

The system...
This truck has a main battery disconnect switch that the firemen turn off when they park the truck in the house and get out. The disconnect switch is a simple heavy duty single pole single throw switch and it is installed on the GROUND side of the system between the frame and the negative battery post.

In addition, there is a charger / inverter installed directly to the battery (does not go through the disconnect) so that when the firemen get out of the truck and disconnect the battery, they also plug in a 110V extension cord. This is extra insurance I guess that the battery will be charged when there is a fire call. This charger / inverter also has a parallel splice that goes directly to a set of 3 power plugs that the firemen use to keep their portable radios charged. This is where the problem comes in...

What's happening is that the little glass fuse that is inside the radio power cord plug keeps blowing everytime they plug the radios in with the battery disconnect switch in the OFF (disconnected) position. If you turn the switch ON (battery connected), you can plug the radios in without blowing the fuse.

Has anybody ever seen anything like this?

I have a system on my truck that is almost identical to what you are describing. What I think the problem is, is that the cutoff switch is cutting off the ground to the batteries, causing too many amps to go to the radio. I would run the positive side of the battery to the cutoff switch instead of the ground, and I think that may solve the problem.

Sounds like the charger is not big enough and putting the truck batteries on line makes up the difference. You could go that route but the batteries will be draining and not charging until the radios are fully charged then the truck batteries will have their turn getting charged.

It's an inverter/charger with shore power. When the truck is plugged into a 110V outlet, the unit is designed to provide the power to the outlets from the 110V line, not the inverter. The inverter is actually not running, but rather is reversed, acting as a trickle charger. This is exactly what my Xantrex system does. My guess as to why it is blowing fuses is because there is a short in the system somewhere which is only showing up when the ground is removed with the cutoff switch. Reversing the cutoff switch to disconnect the hot side rather than the ground side should eliminate the short from blowing out the system.

These systems are great when they work, and are a PITA when they don't. They are extremely complex. Much more than a standard inverter.

Oh, OK. I know nothing about those.

Not sure what year, make and model it is. E One is in Ocala FL and 10-8 fire supply handles Pierce. Most of those folks will take calls and help you troubleshoot over the phone. The trucks I have operated have always had a battery switch to allow either one of the two deep cycle batteries to be turned on independently, both on at the same time or both off. Operating mobile radios, emergency lightbars, scene lights, interior lights and a variety of different chargers usually requires a battery bank. Firehouse.com has a forum that you can post in if Rev's diagnosis is incorrect. I doubt it it but it's possible. If it catches on fire you know who to call right? Ask Rev about his avatar..He's a pretty darn good firefighter. :wink:

http://forums.firehouse.com/forumdisplay.php?f=274

We had a similiar problem and removed our radio chargers in the truck and have a charger bank next to the alarm printer. Our 800 mhz radios have pretty dependable batteries. And we still keep 2 per person stored in the cab and rotate them out every day. Too much electronic crap on board now a days so it doesn't hurt to reduce the load. BOL

Disconnect switches are usually mounted to a metal part of the truck.

The reason the disconnect switch is on the ground side of the system is that in case of a failure of the insulation inside the disconnect switch, the result will be an inability to disconnect, rather than a dead short of the hot side to ground thru the body of the disconnect switch.

To be honest, the main reason I'm saying to swap the positive and negative on the breaker switch is because that's the way mine came from the factory. :lol:

And if the switch experienced a failure of it's isolation of the hot side vs. the grounded metal that it's mounted to, what do you think the result would be?

If the disconnect switch is mounted to a non conductive material, such as a fiberglass body panel, then this is irrelevant, but if the switch is mounted to metal, then it's a valid concern.

I don't think the problem is in the switch. Like I said, I've got one of these on my truck, and there's no way that it is a bad switch. These cutoff switches are massive, and they are completely mechanical.

Here's what the one on my truck looks like.