Safe Working On 12v Systems

[Altinatom]

Hi everyone
I'm a newbie on 12v systems, but learning fast, so apologies for asking questions that may seem stupid to experienced users.
When testing circuits is it OK to pull and re-fit fuses with the system live - I'm concerned it seems crude and might cause damage?
What is the best way to isolate the complete 12v system when working on it, bearing in mind the starter battery and solar panel are connected to it as well as the leisure batteries?
Is it worth fitting isolation switches to the main appliances, e.g. fridge, heater, solar panel - I have been told that the pcb's can sometimes get into a confused loop and that switching the supply on and off, like a PC, will clear it?
TIA

 

[huck999]

Hi,
In a word - Yes.
Its usually safe to play with 12v. Just remember that, unlike house mains, there are high amp loads on many devices. eg at mains voltage 100w pulls only about 0,5A whereas 100W @12v pulls about 8 amps. This means that, whilst safe to you, (generally it is considered safe to work on voltages under 50v) the inrush of current (amps) can cause a lot of damage to appliances. This inrush can cause the fuse you are replacing to fail (blow). But thats what they are designed for.
In that case, turn off the item you want to pull the fuse from (or install) to prevent any possible inrush. However, it is usually not required. A good fuse puller or needle nose plyers will keep your fingers away from any arcing that may occur. Sparking is quite usual as the 12v rushes into the load when the item is switched on. Generally the bigger the wire, the bigger the load and the bigger the spark. You have probably noticed the spark when connecting a car battery
Most motorhomes have a 'black box' that regulates the charging of the batteries and provides 12v to the leisure systems. These usually have a button by the habitation control panel that will kill all 12v to the habitation, except for the important stuff like the fridge and the heaters etc. Buttton top right in this illustration.

To kill the fridge etc.,, you will need to switch off the 'black box' but first make sure that you are not connected to mains power.
These two switches, when turned to off, may also reset any dodgy pcb's as you put it.
However, to kill all power completely, you will also need to disable the solar system and batteries.
Assuming there are no inline switches: ALWAYS disconnect the solar from the charge controller first (And reconnect last!). Make sure that if you remove any bare ends of cables to make them safe by wrapping in tape etc. They will be 'live' - either from the solar panels or the battery. You can now go ahead and remove the NEGATIVE from the leisure batteries and, if required, from the engine battery. The van will now be electrically safe with no 12v power or 230v - but the beer in the fridge will be getting warm!
Now get a multimeter or other 12v tester and check that everything really IS dead.
After doing all that, definitely wire in a 2-pole switch between solar panels and controller, and add a suitable fuse or mcb on the positive line to the leisure batteries. It is best to completely remove the negative cable from the battery rather than using an in-line switch. If you have more than one leisure battery, disconnect all negative leads in case one is still wired to the chassis somewhere.
As for the rest, lights etc, they are usually switched at the habitat control panel or the 'black box' and so do not really need inline switches.
Also, there is usually no need to remove fuses to test them. One of these Fuse tester will do the job in situ as the blade fuses have a contact on the top onto which you can push the tester.. Green led - good , red led - bad

Next time you have a few hours of leisure, check out the many excellent videos on youtube. (electroboom, if you like a laugh)
The black box is usually an elecrobloc 99 or similar. This is typical in most motorhomes.
Good luck with your learning.
Come back to forum if you have any problems - there are many real experts (unlike me) on here, who are only too happy to help fellow members.

 

[Altinatom]

Many thanks for your comprehensive response, full of lots of info I was not aware of. Cheers

 

[autorouter]

As huck999 says, 12V won't give you an electric shock. But one thing to watch out for is that a lead-acid battery can push out a massive current (amps) if the positive is somehow short-circuited to the negative. It can make wires red-hot and cause a fire. That's why it's essential to have every wire fused, so it can blow to stop the dangerous overcurrent. Also watch out for your spanner catching earthed metalwork when tightening nuts on positive fuses and terminals.

 

[Altinatom]

Thanks

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