Blown fuse

[ScottM]

I have one fused item in my panel. The old analog clock.

A year and a half ago the fuse blew while I was flying to Florida. I was in IMC and know the rules of having a real clock I immediately declared an emergency and made a safe partial-panel (no clock) approach

Ok I actually got it fixed at my next stop.

But after the fuse was replaced a year and a half ago it happened again.

If something was failing I would have expected it to happen sooner. I doubt it is a power surge as the fused power line goes straight from the battery to the clock and batteries are great capacitors (filters), not to mention clocks hardly pull that much current.

Anyone run into this type of annoying infrequent ghost of an issue on Cherokee clocks?

 

[CJones]

 

[taters]

what amp fuse was put in ? inspect the wiring if you can and look for place where the wires could be chafing.....otherwise those glass fuses do just go sometimes... maybe try a slightly higher fuse.

 

[mikea]

..
Anyone run into this type of annoying infrequent ghost of an issue on Cherokee clocks?

No. When my Cherokee clock stops I wind it up.

(It's actually not working now. I should take it out and get it fixed.)

 

[gprellwitz]

No. When my Cherokee clock stops I wind it up.

(It's actually not working now. I should take it out and get it fixed.)

Actually, you should start flying it!

 

[nyoung]

Is the fuse cable tied or otherwise supported nicely? It could be mechanically breaking vs failing due to electrical overload.

To really figure out what is going on, you could wire an ammeter in series with the power line.

 

[Skip Miller]

To really figure out what is going on, you could wire an ammeter in series with the power line.

If it is an intermittent overload, you would have to be a lot luckier than I am to see anything abnormal. Besides, you are supposed to be looking outside!

-Skip

 

[gismo]

I have one fused item in my panel. The old analog clock.

A year and a half ago the fuse blew while I was flying to Florida. I was in IMC and know the rules of having a real clock I immediately declared an emergency and made a safe partial-panel (no clock) approach

Ok I actually got it fixed at my next stop.

But after the fuse was replaced a year and a half ago it happened again.

If something was failing I would have expected it to happen sooner. I doubt it is a power surge as the fused power line goes straight from the battery to the clock and batteries are great capacitors (filters), not to mention clocks hardly pull that much current.

Anyone run into this type of annoying infrequent ghost of an issue on Cherokee clocks?

Can't help with Cherokee experience but clocks and fuses are pretty similar across the brands.

Clocks are fairly low power items although really old electric clocks pull a fair amount of current intermittently compared to modern clocks which have a very low constant current draw. So one possibility is that you simply have the wrong fuse. The main purpose of the fuse is to prevent fires if a wire shorts and the fuse size for that is based on the wire size. I'd be surprised if the wiring for a panel clock was smaller than 18 or 20 gauge (the gauge should be printed on the wire if you can get at it to read). The max current for 20 gauge is 7.5A (bundled wire) and for 18 it's 10A and you could try anything up to those ratings safely. You can also tell a lot by the appearance of the blown fuse element if it has a glass envelope. A slight overcurrent will usually cause the element to sag and open a small break. A short circuit which causes much higher current than most fuses are rated for will evaporate a large portion of the element and leave much of the envelope darkened. A mechanical failure, especially with a light load like a clock will leave most if not all the element intact with little or no darkening of the envelope.

Without seeing the actual fuse my guess is that you have a chaffed wire that's shorting to ground somewhere. You might try pulling/wiggling the clock wire to see if you can blow another fuse. Visual inspection of the wire may be difficult but sometimes that will turn up an obvious problem, arcs tend to leave a signature behind. It's also remotely possible that the clock has an internal intermittent short. I can't see much potential for surge to be causing your problem.

 

[ScottM]

Can't help with Cherokee experience but clocks and fuses are pretty similar across the brands.

Clocks are fairly low power items although really old electric clocks pull a fair amount of current intermittently compared to modern clocks which have a very low constant current draw. So one possibility is that you simply have the wrong fuse. The main purpose of the fuse is to prevent fires if a wire shorts and the fuse size for that is based on the wire size. I'd be surprised if the wiring for a panel clock was smaller than 18 or 20 gauge (the gauge should be printed on the wire if you can get at it to read). The max current for 20 gauge is 7.5A (bundled wire) and for 18 it's 10A and you could try anything up to those ratings safely. You can also tell a lot by the appearance of the blown fuse element if it has a glass envelope. A slight overcurrent will usually cause the element to sag and open a small break. A short circuit which causes much higher current than most fuses are rated for will evaporate a large portion of the element and leave much of the envelope darkened. A mechanical failure, especially with a light load like a clock will leave most if not all the element intact with little or no darkening of the envelope.

Without seeing the actual fuse my guess is that you have a chaffed wire that's shorting to ground somewhere. You might try pulling/wiggling the clock wire to see if you can blow another fuse. Visual inspection of the wire may be difficult but sometimes that will turn up an obvious problem. It's also remotely possible that the clock has an internal intermittent short.

I also have not ruled out mechanical failure of the fuse itself. At about one per 18 months it may just be that simple. I have been toying with the idea of a clock upgrade anyways. It would be nice to have a digital clock/timer instead of the old analog model. Installation would also require rewiring and avoid the whole issue of tracking down a potential bad wire.

 

[nyoung]

Very true, but it would be an interesting exercise to see if the average current is more than expected. Although I don't know for certain, it seems that a clock should be << 1 amp. If the thing is pulling 4.5amps constantly, then something is wrong.

As an aside... With gps, cell phones, west bend kitchen timers, wristwatches... It's a shame we have to have a clock in the panel.

If it is an intermittent overload, you would have to be a lot luckier than I am to see anything abnormal. Besides, you are supposed to be looking outside!

-Skip

 

[wildwobby]

Very true, but it would be an interesting exercise to see if the average current is more than expected. Although I don't know for certain, it seems that a clock should be << 1 amp. If the thing is pulling 4.5amps constantly, then something is wrong.

As an aside... With gps, cell phones, west bend kitchen timers, wristwatches... It's a shame we have to have a clock in the panel.

Are analog clocks really that inefficient? I know a digital clock shouldn't be using more than something in the mA range.

 

[gismo]

Very true, but it would be an interesting exercise to see if the average current is more than expected. Although I don't know for certain, it seems that a clock should be << 1 amp. If the thing is pulling 4.5amps constantly, then something is wrong.

Average current should be less than 25-50ma, otherwise the clock would kill a battery completely in a few weeks of inactivity. The older analog clocks were essentially a windup clock complete with mainspring and escapement. An electric motor would wind up the mainspring occasionally. The result was a low average current but high peak current. Newer analog clocks use an extremely low power motor to drive the clock directly.

 

[Dan Thomas]

Average current should be less than 25-50ma, otherwise the clock would kill a battery completely in a few weeks of inactivity. The older analog clocks were essentially a windup clock complete with mainspring and escapement. An electric motor would wind up the mainspring occasionally. The result was a low average current but high peak current. Newer analog clocks use an extremely low power motor to drive the clock directly.

Some of those had a solenoid that would the spring every few minutes. You'd hear it click. The solenoid draws more current than the fuse is rated for, IIRC, and the parts manual might be specifying a slow-blow type of fuse. The answers to such problems are often in the manuals.

Clocks and Hobbs meters use small fuses for good reason. Their systems are still hot after the master is turned off, and in a forced landing that goes bad a larger fuse could provide enough power to generate enough heat or spark to ignite spilled fuel. A small fuse will blow quickly when the wiring is torn and shorted and reduce the likelihood of fire. Installing bigger fuses increases the risk.

Dan

 

[Ghery]

Average current should be less than 25-50ma, otherwise the clock would kill a battery completely in a few weeks of inactivity. The older analog clocks were essentially a windup clock complete with mainspring and escapement. An electric motor would wind up the mainspring occasionally. The result was a low average current but high peak current. Newer analog clocks use an extremely low power motor to drive the clock directly.

Some of those had a solenoid that would the spring every few minutes. You'd hear it click. The solenoid draws more current than the fuse is rated for, IIRC, and the parts manual might be specifying a slow-blow type of fuse. The answers to such problems are often in the manuals.

Clocks and Hobbs meters use small fuses for good reason. Their systems are still hot after the master is turned off, and in a forced landing that goes bad a larger fuse could provide enough power to generate enough heat or spark to ignite spilled fuel. A small fuse will blow quickly when the wiring is torn and shorted and reduce the likelihood of fire. Installing bigger fuses increases the risk.

Dan

On the other hand, the clock in the club's 182 only runs when the master switch is on. You have to reset it each time you fly. Really annoying, but you don't have to worry about it killing the battery.

 

[Dr. O]

My Hobbs meter blew a fuse... Looked things over, shrugged, replaced the fuse and moved on... A few weeks later it blew again... This time I went on a search and destroy mission, tore off the glare shield and delved into the Stygian depths behind the panel (the horror, the horror) and found a splice in the power wire that had the tape removed (???) and the bare metal was exposed... Fixed that, smiled and went flying... A few weeks later it blew again - whut the hey? After a long and miserably hot afternoon spent tracing the power wire to the meter I followed it out to the nose where it looped behind the coaxial cables from the glideslope antenna on the nose cone and came back to the battery - at this turn in the wire the insulation had gradually chafed and exposed bare wire... Fixed that and no problems since...