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Glass fuses on the switch panel...


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21 minutes ago, Hartlebury lad said:

Its a 1996 boat self fitted out with several previous owners - no manual of course, and i am not convinced that the ones currently in the circuits are correct. Just because they haven't blown doesn't mean they are correct?

They have to be small enough to protect the cable but hopefully you will be able to fit ones large enough for the load they have to carry. The labels give no clue to what the load would be, for example, toilet light, LED, Fluorescent, filament, built in fan. Pump, how big?

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Water pump is a common or garden  Shurflo ( i know ....!)

Bathroom pump out is a Whale Gulper

The 8 cabin lights are all LED, as is the single bathroom Light.

The toilet pump is a Thetford C200? and 3 rear lights are LED

Horn is a standard 12v car horn (long cable run), as is the front spot.

C Heating pump is a tiny Chinese Import jobbie, fed by the calorifier. 

 

 

 

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Ignoring it being a self fitout because we can't know what the fitter knew or cared.

 

The smallest cable is likely to be 2 sq mm CCSA and that has a minimum rating of 14 amps and may be more. In fact some cables are likely to be larger to minimize volt drop. So I think that you can probably safely fit 10 amp fuses. I have no idea if this will be enough for the water pump, but probably will be for the rest.

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First check the physical size - usually 20mm (long by 5mm diameter), or 1/4 inch (diameter by 1 1/4 inch long) - often referred to by first quoted dimension hence the reversal.

To determine the current rating you need to consider/investigate what they are feeding - water pumps are often marked with the fuse rating on the body of the pump, Tunnel lights are often 55W halogen bulbs and will theoretically require 55 (W) / 12 (V)  = fuse rating 4.6 (A) and then add a little bit to get to the nearest available rating HOWEVER this assumes that the cable runs are suitably sized for the length - I would not be surprised to find a 5A fuse would not be sufficient for the tunnel light due to voltage drop over a long run even with suitably sized cable. Cabin lights - add the wattage of ALL the lights protected by the fuse together and calculate as above - eg 8 x 2W LEDs = 16 W - a 2A fuse would be sufficient, for 8 x 5W LEDs = 40W you would need 5A etc.   Remember the fuse is to protect the cable. As Tony said 10A should be OK for the lighting, the pumps you need to check individually, The horn is a high current device, and a long cable run BUT is only used for relatively short periods (5 - 10 seconds ?) - would probably need 15A assuming suitably sized cable. Undersized cable increases the current to compensate for voltage drop.

 

The existing fuses should be marked on the metal cap with their rating though it can be difficult to read - I often need a lens and good lighting to read it - oblique lighting may help.

 

springy 

Edited by springy
add "marking" para
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Glass fuses come in different physical sizes,, different current ratings, and different response times. The older imperial size, 1 1/4" x1/4" (often now called 32mm x 6.3mm) has been largely superseded by the 20mm x 5mm size.  

Response times (slowest to fastest) are:

Slow blow (anti-surge) (T)

Medium delay,

Fast (F)

Very fast.

 

The response time will be stamped on the end caps.

 

Fast and very fast types would be used for sensitive electronic equipment like test meters.

 

For loads subject to momentary current surge at switch-on, such as  tungsten lamps and motors, the anti- surge type is normally preferred. 

 

Glass fuses are now usually rated by the current they can carry without blowing, and this is the present international standard.  Automotive-type blade fuses are normally  still rated in the old way, by specifying  the current at which they will blow, which is typically twice the carrying current. I have come across  glass fuses for automotive use that are still rated this way, and have some vintage glass fuses which are marked with both current ratings.

 

For the mains-rated glass fuses that you get from electronics distributors, 1 1/4" fuses are commonly available in ratings up to 15A or 16A, 20mm to only 10A, but there are exceptions, especially for fuses for lower voltages.

 

 

 

 

Edited by Ronaldo47
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At £8 for 100 assorted fuses (even at Halfords prices) you could perhaps experiment with the lowest rated fuses , working upwards as required, on the circuits where the amps required is uncertain

image.png.372b245fdd305b8ad27d118e5197eec9.png

 

image.png.bf307c268f007af8485efaf554324f56.png

  • Greenie 1
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A decent motor factors should stock glass fuses, perhaps take one with you to get the right size so you can get some spares.

 

You could use a DC amp clamp to work out the maximum draw on a circuit but beware, fuses are there to protect the wiring, over the years devices may have been added to circuits without thought.  You should also calculate (from the wire gauge) how may amps are safe for each circuit...

 

 

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23 minutes ago, springy said:

First check the physical size - usually 20mm (long by 5mm diameter), or 1/4 inch (diameter by 1 1/4 inch long) - often referred to by first quoted dimension hence the reversal.

To determine the current rating you need to consider/investigate what they are feeding - water pumps are often marked with the fuse rating on the body of the pump, Tunnel lights are often 55W halogen bulbs and will theoretically require 55 (W) / 12 (V)  = fuse rating 4.6 (A) and then add a little bit to get to the nearest available rating HOWEVER this assumes that the cable runs are suitably sized for the length - I would not be surprised to find a 5A fuse would not be sufficient for the tunnel light due to voltage drop over a long run even with suitably sized cable. Cabin lights - add the wattage of ALL the lights protected by the fuse together and calculate as above - eg 8 x 2W LEDs = 16 W - a 2A fuse would be sufficient, for 8 x 5W LEDs = 40W you would need 5A etc.   Remember the fuse is to protect the cable. As Tony said 10A should be OK for the lighting, the pumps you need to check individually, The horn is a high current device, and a long cable run BUT is only used for relatively short periods (5 - 10 seconds ?) - would probably need 15A assuming suitably sized cable. Undersized cable increases the current to compensate for voltage drop.

 

The existing fuses should be marked on the metal cap with their rating though it can be difficult to read - I often need a lens and good lighting to read it - oblique lighting may help.

 

springy 

I think you need to check Ohms Law again

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45 minutes ago, MartynG said:

At £8 for 100 assorted fuses (even at Halfords prices) you could perhaps experiment with the lowest rated fuses , working upwards as required, on the circuits where the amps required is uncertain

image.png.372b245fdd305b8ad27d118e5197eec9.png

 

image.png.bf307c268f007af8485efaf554324f56.png

They look like the 1 1/4" size. Being from Halfords, I would expect them to be rated by their blowing current rather than their carrying current but it would be worth while checking. I used to get my fuses from Maplin, where their glass fuses were mains-rated,  and rated by carrying current.

Edited by Ronaldo47
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18 minutes ago, ditchcrawler said:

I think you need to check Ohms Law again

 

Ohms law relates Voltage, Current and Resistance, I'm using power calculation, Power (Watts) = Volts x Amps, rearranged as Power / Volts = Amps

 

springy

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Just now, springy said:

 

Ohms law relates Voltage, Current and Resistance, I'm using power calculation, Power (Watts) = Volts x Amps, rearranged as Power / Volts = Amps

 

springy

But your headlight is just a resister that gets very hot so less applied voltage = less current, motors on pumps its hard to say as less voltage can mean a slower rotation so possible less back EMF which may lead to more current as its not purely resistive. With something like an invertor you are of course correct

  • Greenie 1
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Tungsten has a non-constant, non-linear temperature/resistance characteristic, meaning that the resistance of a tungsten lamp bulb filament  at room temperature is about one-tenth of its resistance when hot and operating at its rated voltage.  Hence the need for an anti-surge fuse, as the current at the instant of switch- on will be around ten times the  operating current at its rated voltage.

 

A tungsten bulb can be used as a rough approximation to a constant current load, because as the voltage reduces, so does the filament  resistance, meaning that the current does nor reduce by as much as it would if the resistance were constant.  

Edited by Ronaldo47
typos
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11 hours ago, springy said:

HOWEVER this assumes that the cable runs are suitably sized for the length - I would not be surprised to find a 5A fuse would not be sufficient for the tunnel light due to voltage drop over a long run even with suitably sized cable.

 

I don't think that is correct, or at least not correct for the reason you state, especially for a lamp which is a resistive load.

 

In the case of a resistive load the resistance of the cable, the thing that causes volt drop, in effect simply adds to the resistance of the bulb so LESS current will flow and the bulb will be dimmer than normal.

 

Where you may be correct, but again not as you state, is in the case of a motor with a cable that has not been sized to minimize volt drop and as its  a self fit out this could well be the case. The reason why excess volt drop on cables feeding a motor is that the slower most motors go the higher their current draw. This is why electric motors produce maximum torque at zero revs, as they accelerate the pulsing magnetic fields inside them in increases their inductance -  ordinary boaters can consider inductance as similar to resistance. So if the volt drop along the cable is high enough to reduce the voltage at the motor enough to prevent the motor reaching its designed speed it will draw more than its rated current. Then it is more likely to burn out, and in the case of a water pump take longer to reach pressure and maybe not automatically turn off

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As an aside, the horn was one of a twin horn set off a Vauxhall Astra GT thingy - I installed them years ago and thought they would be the business, but when i pushed the switch, they barely flickered. I took one off and all was fine.

Of course, i forgot to do it whilst the engine was running, which is when you need the horn anyway! No doubt, they would have worked fine under those circumstances.

 

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2 minutes ago, Hartlebury lad said:

As an aside, the horn was one of a twin horn set off a Vauxhall Astra GT thingy - I installed them years ago and thought they would be the business, but when i pushed the switch, they barely flickered. I took one off and all was fine.

Of course, i forgot to do it whilst the engine was running, which is when you need the horn anyway! No doubt, they would have worked fine under those circumstances.

 

 

I bet they made a grunt or a click and then silence. Horns are an inductve load because they keep turning their coil magnet on and off.

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