12v Fridge Element

[BruceS]

I'll let T1 or others comment on that but I seem to remember that those particular fuse holders are the problem rather than the actual fuse.
I nearly lost a truck to those particular type fuseholders once. In a Mercedes Truck no less!
I'd love to see my "Watts Meter" plugged in to see EXACTLY what that fridge is drawing.
Where are you Steve? Qld?

[Craig]

The problem with those blade fuses is they don't like constant high currents. I had a similar problem on my solar set up where the fuse melted but did not blow. I changed them out for the short stop breakers the same as in the link that admin put up. Be aware that if you do go down that path there are 3 types, they are auto reset, semi-auto reset and manual reset. I prefer the manual re-set.

I know that these elements are about 275 watt, so a 25 amp fuse will be barely big enough. The problem will be when and if the voltage starts to drop the amps will rise. 275 watts divided by 12 volts = 22.9 amp. 275 watts divided by 11 volts = 25 amps. So as you can see as your voltage drops the worse off you are. I thought they were fitting 30 amp fuses now. Make a couple of calls to dometic.

Craig

[PandJ R away]

I can't add anything to the fridge discussion but I support the comments about the small blade fuse. I had a 4 fuse block on the first setup for the house system in the Coaster. One of the fuses was on the line form the solar panels to the regulator.

It got so hot it melted the fuse block which dripped into the regulator. Within the melted plastic a short was created which burned a 1/4 deep impression in the mdf mounting board and filled the inside with smoke and plastic soot. We could easily A have lost the bus. A lesson learned.

John

[T1 Terry]

Missed this one somehow, those little blade fuses are a problem anywhere above around 15 amps, the contact area on the fuse to holder is too small. The spring bit the fuse pushes into is rounded, the actual contact area is quite small. If you plaster it with a conductive grease like Alminox, that would inprove the contact area, but I wouldn't he using that size fuse for that much load, the AG5 fuse holders and fuses the car stereo boys use are the best for that sort of load, the fuses range from 20 amps through to around 150 amps. After that it AML wafer fuses.

[Craig]

Unfortunately the AG5 fuse holders and fuses are still simply that, "a fuse". The beauty with the "Shortstop Breakers" is they can be reset. For my money they are a far better choice.

[oldtrack123]

I know that these elements are about 275 watt, so a 25 amp fuse will be barely big enough. The problem will be when and if the voltage starts to drop the amps will rise. 275 watts divided by 12 volts = 22.9 amp. 275 watts divided by 11 volts = 25 amps. So as you can see as your voltage drops the worse off you are. I thought they were fitting 30 amp fuses now. Make a couple of calls to dometic.

Craig

Hi

Resistive loads do not follow that rule
if the element is rated at 275W@12v it will draw 22.91A AT 12v
but with 11v it will only draw 20,99A
The resultant Wattage is now 230.9W a loss of 16% in power input
That is why maintaining rated voltage is important with absorption fridges


PeterQ

[oldtrack123]

Missed this one somehow, those little blade fuses are a problem anywhere above around 15 amps, the contact area on the fuse to holder is too small. The spring bit the fuse pushes into is rounded, the actual contact area is quite small. If you plaster it with a conductive grease like Alminox, that would inprove the contact area, but I wouldn't he using that size fuse for that much load, the AG5 fuse holders and fuses the car stereo boys use are the best for that sort of load, the fuses range from 20 amps through to around 150 amps. After that it AML wafer fuses.

Hi Terry
YES

PeterQ

[T1 Terry]

Circuit breakers have a few issues of their own. We had a few dramas with a 125 amp one that was not consistant with it's 125 amp trip point, it would trip early if the load was close to 100amps for more than 15 mins, but not trip till near 200 amps on a short term load. There is also problems with arcs forming across the contacts and welding them together, usually where the trip mechanism looses some of it's tension so the break is slower than designed, this often happens when a circuit breaker runs close to it's trip point for an extended period.
Fuses aren't perfect either, voltage rating is also important when DC is involved, forming a plasma arc across a rapidly blowing fuse can lead to a fire all by itself. Usually slow blow fuses ae used if this is a possible issue, they actually contain a non conductive powder so if the fuse blows any arc is imediately quenched.

[oldtrack123]

Circuit breakers have a few issues of their own. We had a few dramas with a 125 amp one that was not consistant with it's 125 amp trip point, it would trip early if the load was close to 100amps for more than 15 mins, but not trip till near 200 amps on a short term load. There is also problems with arcs forming across the contacts and welding them together, usually where the trip mechanism looses some of it's tension so the break is slower than designed, this often happens when a circuit breaker runs close to it's trip point for an extended period.
Fuses aren't perfect either, voltage rating is also important when DC is involved, forming a plasma arc across a rapidly blowing fuse can lead to a fire all by itself. Usually slow blow fuses ae used if this is a possible issue, they actually contain a non conductive powder so if the fuse blows any arc is imediately quenched.

Hi Terry
Thermal Circuit breakers ALL have a current /time curve
The time curve performance should be in the specs
Fuses are thermal devices they also have a current/ time curve
The only breakers that will trip at figure close to its rating are magnetic,will trip as soon as that point is reached
Thermal magnetic OLCBs are also available which combine a time delay plus a definite over current ,almost instantaneous trip point
Of course care should be taken using AC olcbs on DC circuits as you have pointed out the arc quenching requirements are vastly different
Fuses come in many forms
Slow blow ,fast blow , standard , HRC[high rupturing capacity, filled with a powder with arc quencing properties ] motor start [similar to slow blow , copes with initial surge current but retains normal rating in the long term]

Each have their application & care is needed to replace like with like.
EXCEPT When they were not right in the FIRST PLACE as in this case
I suspect another install by untrained monkeys


PeterQ

PeterQ

[Kelvin12]

Interesting discussion here. Learning a lot from it. Is it possible to post some photos or links to the various items. Would help to recognize a lot of the items under discussion.

Thanks to everyone here, this is important stuff.

Dirk