Avoided Electrical Meltdown

[Marc14857]

Just wanted to share this weekend's close call with shorepower AC connection. We were onboard tied up to the slip with a single 30 Amp Shorepower line coming into the boat. We had the water heater powered up all day as well as the battery/inverter with no apparent problems. Both of these are on AC Line 1. We started the microwave on the same bus to heat some water up (done this many times before) and while the microwave did run we started to smell that acrid odor indicative of something electrical burning. No breakers tripped but I did manually trip the main AC breaker onboard. Opened up the AC distribution panel at the Nav station and while I did not see any smoke the acrid smell was definately coming from behind the panel and there was definately some fairly significant heat coming from behind the panel. We let everything vent and air out and didn't see any apparent wire damage though did not have a great view of all the wires under the bundle. Thinking that the problem was limited to the microwave circuit, we reennergized the AC distribution panel powered up the battery/inverter and hot water heater with no apparent problem. We left the microwave deenergized and was planning on having the yard look it over the next day. Next morning some dockmates came over and we deceided to do some trouble shooting. We opened up the AC panel which was live and switched on the microwave with no discernable problem. When the microwave actually started however we saw sparks flying somewhere underneeth the wiring bundle in the vicinity of the neutral busbar on the AC distribution panel. Turned the microwave off and the sparking stopped. Started to remove some wires to get a clearer view of what was sparking which turned out to be unrelated to the microwave. Apparently there is a larger gauge white neutral wire that is the neutral return from the AC distribution panel busbar for all the circuits that had become loose and was arching whenever the load on the bus increased above a certain threshold. It must have been going on for quite some time as the wire was severly burned as well as some of the neighboring wires. We also found some other terminations to the busbar that had become loose over time. Had the yard replace the burned terminations and tighten up all the connections and everything is fine now.

I suspect that no one ever checks the electrical terminations on the backside of their AC distribution panels and just opening up the panel may cause some wires to become loose. Not sure what if anything to recommend here but wanted you all to perhaps tuck this away somewhere in case you are inclined to check the terminations or ever are in the position where you have that acrid electrical smell on your boat.

 

[Rick D]

Thanks for sharing that experience! Good reminder.

 

[Chief RA]

Marc: while wiring my boat I found a loose common on the buss bar as well. Fortunately it had not caused any burning but of course I tightened it anyway. Interesting thing about it though is I suspect it has had this since it was built since the reason the screw was not tight is it was bottoming out on the fiberglass bulkhead where the bar was bonded before getting tight on the lug. Chief

 

[Benny17441]

The question is, why didn't a breaker trip?

 

[Marc14857]

My guess is that the current draw was not high enough to trip the breaker though the connection was sparking due to the loose connection. None of the individual load breakers nor the main AC breaker tripped.

 

[wufibugs]

Am I correct in understanding is that most of these breakers are tripped by the heat created by resistance in the wire being protected? As I understand, they are triggered by the dissimilar metals principal that controls the "darkness" setting on a toaster. Since the wire was arcing well before the breaker and transferring that heat elsewhere, the wire probably did not get hot enough to trip the breaker, which is really scary.
Curious, Marc and Chief. What kind of connection had come loose? Screw down, spade, ring?

 

[Chief RA]

Richard: It was a screw down on a spade connector. All the other screws had 2 or more connectors and tightened fine but this one had only one and could not tighten down.
With reference to the overload bimetals tripping: excess current through the metals causes the plate to invert and release the electrical contact. Not wire heat, wire excess current is the tripping factor causing heat within the bimetal unit. Its likely the reason it did not trip is there was not excess current, only localized arcing and heat at the poor contact point. This of course can cause a fire though!
Hope this helps. I taught this stuff for US Navy and Army and Butte College.

Chief

 

[heritage]

If you had a main panel breaker that combines a normal thermal type breaker with a GFCI type then it would have tripped when the neutral wire was sparking. The GFCI circuit would have detected a difference in current flowing in the hot and neutral wires. Also, this type of panel breaker is supposed to help protect swimmers near your boat if you have an ac fault. This article by Blue Sea may be helpful: http://www.bluesea.com/resources/577

 

[Chief RA]

Good point Heritage. I have GFCI's installed in my boat and yes, they probably would have tripped. Chief

 

[Parrott]

As a Industrial Electrician for 40 years I will say any (any) loose connection you have will become a problem at some time, so always stop, turn off power and check you're connections, and you'll be trouble free.