Blown fuses.

[Maynard Rupp]

Those of us that are lucky enough to have those stupid fuses to protect our shore power inlets are tearing out what little hair we have left. Our '86 36C, "Unity", blows them on the AC line like popcorn. We switched to the slow blow ones and that has helped. Now that our boat is in Florida, we need to leave at least one AC unit on all the time. In Detroit we had to put the boat in winter storage by now, so we are still better off. My son watches "Unity" like a hawk, but had to leave the country last week. He returned to find one of the AC line fuses blown. I fully understand why they blow when both units and the water pump are on line. One look at the amp. gage explains why that happens, but why would that stupid fuse blow with only one 16000 BTU AC unit on line? The diver also told us that we are eating zincs kinda fast. Oh man, I sure ain't used to this. The first zincs looked good after 20 years in fresh water. I changed them before I left for Miami on Labor day just because. I guess I am trying to find a relation between our fuse problem and our zinc problem. These boats do not have an isolation transformer. on the AC line. They do have a seperate 30 amp. connector for the AC though. Hatteras says that's OK as the power is seperate from the ship's, which has an isolation transformer. I am not convinced.

 

[captddis]

The fuses are a poor idea and fail well before the amp rating is reached. Not only the fuses fry but so does the wire on the back of the fuses. i WOULD REWIRE FOR 50 AMP. The 30 amp just aint enough. I know of a couple 36s that melted the wire between the fuses and the panel! Put breakers in and no more problems. Done it on dozens of Hatts. And others. Dave

 

[SKYCHENEY]

I have those fuses and have only ever replaced one. But, I have two 50a 240v cords and two isolation transformers. It would seem that there has to be a draw exceeding the limit of the fuse for it to blow. Are you sure that the unit didn't freeze up or malfunction in some way?

 

[TedZ]

That's why they blow! Very funny.

All of the world's problems can be solved if you have a computer!

Buy a logger http://www.onsetcomp.com/index.html

You have to call, or play a little on the site to get a deal for under $100. Buy a current transformer or get one with the logger. They are 0 to 5 volts output. Hook it up and when a fuse blows hook the logger up to a computer (cables and software included) and now you can see what has happened. I think you will see you are getting high current spikes for a long time (many milleseconds). Find the problem which is likely the AC unit and fix it. Confirm fix with logger. If you need to re-wire, at least you know what the max inrush current is...pretty important info.

Next problem, buy a temperature probe, a pressure one, anything you want to monitor and you are in business for $10 to $20 and you can stop guessing.

In the long run, you will save time and be a lot more happy!

Naturally, there are multi channel, fast loggers that cost more but we don't need them as a general rule. A pack of cigarette sized 4 channel logger is all you need.

Ted

 

[Trojan]

Check your line voltage with a volt meter. If you have a panel meter don't trust it. Use a separate meter. It sounds like you have low shore power. The lower your shore power goes the higher the amperage goes. I had this trouble this past summer. I ended up burning up an AC unit, a power cord and the ships entrance plug along with the feeder wires. Lots of smoke. I have no fuses. It was the Marina's fault for low power, but I should have known better. But I wanted that AC.


BILL

 

[REBrueckner]

There are several possible causes of blown fuses.

As noted, low shore power voltage can cause the current flow to increase and that may be a contributing factor. If you have a clamp meter, read the current flow with first one and then two ac units running...see how the actual current compares to the fuse rating your'e using and the ac rating. Maybe a slightly larger fuse is all that's required, but not if the connection is oxidized...

Oxidation (slight corrosion) of the fuses and holders often causes heating at perfectly normal current levels. As oxidation occurs, it creates irregularities in the fuse to holder connection..where a contact point occurs flows and heats the fuse enough to melt it. Be sure the the fuse holder tightly grips the fuse ends. If not, remove the fuse and crimp the holder with a vice grip or similar tool. Rotating each fuse in its holder periodically scrapes off oxidation and assures good contact. Scrubbing both contacts with bronze wool will also help make good contact. With just one air coniditioner running, for say a half hour, check the fuse temp with an infrared thermometer gun...see how hot it's getting. A loose wire connection or partially broken fuse holder at the fuse can also cause heating there.


Disappearing zincs may or may not be related to the above. Probably not. Testing for stray currents is easily done via a corrosion test meter if you have one available. You can also use a voltmeter or ammeter. If you disconnect a bonding wire from an underwater piece of metal, insert a voltmeter and see what ac and dc readings you get. With zincs connected you should see about 500mv dc..that is, about 1/2 volt or so..Much more more and you have an electrical leak of some type. But it would have to be massive to cause a fuse to blow..not the most likely place I'd start. Zincs often last less than a year, especially when a boat is run a lot and wear and tear uses more zinc than protection. Hanging a supplementary zinc over the stern which is connected to your bonding system, or using an extra zinc on each prop shaft is another approach.

 

[TedZ]

I don't have any stock in Hobo but if you tape a $.89 thermocouple to the center of a fuse and use a CT for the ampacity you can track the whole situation. You are going to stand there a long time watching for the problem with a clamp on amprobe.

Naturally, if the voltage is low, as has been said a lot of times on this forum, the ampacity is going to go up. Not only will it go up, it will spiral up due to the resistance going up with temperature hence drawing more current.

This is a bad scene. Avoid low voltage even for a little while.

Ted

 

[Freebird]

You have to be careful where you dock, it looks like you may have picked a queer electron or two and now they are blowing fuses.

Alright mods and tech, tech, tech, guys. If I had been clever enough to come up with this "helpful hint", it would have been zapped for sure.

Chris is a smart fellow, but like myself, he also enjoys a good poke at the serious side of things. Do I find this unacceptable, no. Do I feel I've been singled out unfairly at times, yes.

Like I've said before, this is not about brain surgery, and I'm happy to hang out on the lighter side of the forum with an occasional venture to this side for more serious matters.

Now, where's the toll-free number for the ACLU?

 

[67hat34c]

how are the connectors for your shore power? We found alot of people on our dock including us having melt downs this summer. alot of it was due to poor connections. use emory paper to clean up the connections and then use a di-electric grease to coat them.

Do you have a galvanic isolator? if not then this could be a cause of your zincs going bad. You may also try using a guppy. (large zinc plate on a grounding wire) sink the guppy under the dock and run the wire to a connector in your cockpit, tie to your bonding system. there may be a boat near you eating your zincs. If yours is under zinced then his boat will eat yours.

 

[Maynard Rupp]

I have cleaned the copper ends of all the fuses with emery paper. I have also removed the entire stainless plate with the shore power connectors. The back side is like new. Because the fuses get very hot sometimes , I suspected high resistance in their holders. I took each apart, and , for the life of me, can't see any wear, corrosion, or any other problems. They are all stainless, and look like new. I bought new shore power cords as the old ones, when used on the AC line, would get warm. I guess what I really do not understand is why they would blow with only one AC unit, (of two), running. I surely realize that when both are running, we are pushing the 30 amp limit very hard. 50 amp shore power, while an excellent answer, would require a complete re-wire. All the wires would be too small. I also don't understand why it is always the fuse that blows. There is a 30 amp breaker on the stand, the fuses, a 30 amp breaker on the ships panel, and seperaqte breakers for each AC unit and the pump. It is always the fuse that loses.

 

[Genesis]

Get out an ammeter and see what you're ACTUALLY pulling. A clamp-around works fine for this.

Inlet fuses are a GODO THING because high-resistance connections in the inlet are likely to melt the fuse before you get a fire. Very, very small deficiencies there count - but you won't know where to start looking until you measure the current actually being pulled.

Realize that 50A/250V is almost four times the wattage of a 30A/120V feed.

 

[Trojan]

The fuse is supposed to get hot. When you reach it's temperature limit the fuse BURNS open. It's NOT like a breaker. When you force the fuse to pass current at its higher limits. 2 AC units. It starts to loose its ability to carry the full load. Then when your single AC starts, the inrush current at start-up is enough to burn the fuse link into. I would use a larger fuse 35 or 40 amp standard fuse, NOT slow blow. Slow blow fuses are BAD. You have the added built in protection with the breakers. If it was a corrosion problem it would occur right away. Not a day or two down the road. Once corrosion of a connection starts it will get worse real quick under heavy load.


BILL

 

[UNIQUE_NAME]

the problem i came across was anything larger than 30a was a physically different size (much larger).

 

[67hat34c]

what about the shore power post on the dock. I would request that the marina service it. might be bad connection in the post. Saw 3 of them melt down on our dock over the summer.

 

[Paul45c]

I'm a total novice with electrical issues, but can anybody calculate the load that those a/c systems SHOULD be pulling? Does it closely approach 30a? If the calc says no, as someone mentioned earlier there could be a prob with one of the a/c units.

 

[UNIQUE_NAME]

based on the cruisair plates plates, 2 16k units should be pulling 26 - 27 amps.

 

[Paul45c]

based on the cruisair plates plates, 2 16k units should be pulling 26 - 27 amps.

Well, then one should be close to half that, right? And you say you're blowing all the time with just one. Doesn't sound like you're trying to pull more through your shorepower cord than you SHOULD be doing, so if everything is clean in the receptacle area, I'd be looking next at whichever a/c unit is the one being run solo and the wiring running to it from the main panel. Again, though, I'm no electrician. Advice-taker beware! Good luck.

 

[captddis]

The 50 A does not require a rewire, just new cables to the panel. You buy 1 new inlet, 1 new cord and a new breaker at the inlet. Problem solved forever!

 

[captddis]

The heat from the fuses also melt the wiring behind the fuses and that aint a good thing!

 

[Maynard Rupp]

The fuse is supposed to get hot. When you reach it's temperature limit the fuse BURNS open. It's NOT like a breaker. When you force the fuse to pass current at its higher limits. 2 AC units. It starts to loose its ability to carry the full load. Then when your single AC starts, the inrush current at start-up is enough to burn the fuse link into. I would use a larger fuse 35 or 40 amp standard fuse, NOT slow blow. Slow blow fuses are BAD. You have the added built in protection with the breakers. If it was a corrosion problem it would occur right away. Not a day or two down the road. Once corrosion of a connection starts it will get worse real quick under heavy load.


BILL

I looked at 40 amp fuses. They are too big for those fuse holders.