Do I need an MCB for a small inverter (120W)

[mascip]

With a small inverter (120W), is it necessary to have an MCB for overcurrent protection?


As the inverter can only produce 0.5A (say, 2A to be optimistic), it should never be able to melt those 1.5mm2 cables (rated 21A).

Am I missing something important?

We do need an RCD for sure. Just wondering about the MCB.

 

[Risteard]

Re: Do I need an MCB for a mall inverter (120W)

Short circuit protection.

 

[mascip]

Re: Do I need an MCB for a mall inverter (120W)

If a short circuit happens between phase and neutral (hence not triggering the RCD), then this small inverter would not be able anyway to melt a cable which can take up to 21 amps. Or am I wrong in thinking that?

 

[telectrix]

Re: Do I need an MCB for a mall inverter (120W)

i'd imagine that the inverter itself contained overload protection. i.e. if overloaded it would cut out.

 

[Deleted member 72666]

Re: Do I need an MCB for a mall inverter (120W)

Inverter and RCD are you sure?

 

[mascip]

Re: Do I need an MCB for a mall inverter (120W)

Always an RCD after an inverter, yes. It's pay off the EU'a Recreational Craft Directive, to protect purple against a 240v short.

Good point telectrix. So an MCB could prevent the inverter's internal overcurrent protection from blowing. But in that case it would need to be a tiny MCB (0.5A, or maybe 1A). Do these exist?

 

[Risteard]

Re: Do I need an MCB for a mall inverter (120W)

i'd imagine that the inverter itself contained overload protection. i.e. if overloaded it would cut out.

Sorry wasn't thinking straight!

 

[mascip]

Re: Do I need an MCB for a mall inverter (120W)

I guess, the best option might be to put a DC fuse before the inverter, with a rating a bit smaller than the one inside the inverter. Hence avoiding to have to open the inverter to change a fuse in case of overload.

 

[David M]

Re: Do I need an MCB for a mall inverter (120W)

You would require short circuit protection for the supply to the inverter regardless of its overcurrent protection. Can you not check with the manufacturers of the inverter about the requirements for the overload protection? You may find that Tel is correct that it has a built in overload function. Failing that, there are plenty of options available to protect the device, fuses, overload devices...

 

[mascip]

Re: Do I need an MCB for a mall inverter (120W)

There is overcurrent protection from the supply, but only to protect the cables, I think it's 10A. If I put 0.5A or 1A it might prevent the inverter from being overloaded. I guess I'll either have to call the manufacturer, or open the inverter.

Thank you

 

[UKMeterman]

Re: Do I need an MCB for a mall inverter (120W)

The supply sides does need proper overload protection, where is the battery, how big is it, what type is it, what are the charging arrangements and so on?
Thanks

 

[mascip]

Batteries are in the engine bay.
It's two 100Ah sealed lead acid leisure batteries.
Charged by the engine alternator and solar panels.
But that doesn't seem very relevant to my initial question?

The inverter is 3 meters of 6mm2 cable away, so that's alright in terms of voltage drop.
The cable goes through a battery isolator switch.
There is a 50A fuse between the battery and the battery isolator switch.
There is a 15A fuse near the isolator switch, on the positive line to the inverter.

My original question was whether I also need an MCB on the 240v side, after the RCD. I still haven't contacted the manufacturer.

 

[Richard Burns]

You really do want some level of protection that can operate quickly in case of a fault unless the inverter itself is fully internally protected.
Fuses seem like the most appropriate and simple method of protection but you could use an MCB if you wanted to, they are reasonably well available from RS, Farnell and the like.

 

[mascip]

Thinking about this again, my thought process right now goes like this:

The role of an MCB on the 240v side is to protect the wires and applicances on the 240v side of the inverter.

To melt the 1.5mm2 cables on the 240v side of the installation, I would need more than 21A of current at 240v, which is 5040W. To produce this amount of energy, it can only be provided by the 12v batteries, which means that there would need to be 5040W/12V = 462A coming through the cables on the 12v side. Which won't happen because current is limited to 15A by a fuse on that side. 15A * 12V = 600W, and 600W / 240v = 2.5A. I won't be able to get much more than 2.5A from the 240v side of the inverter.

As a consequence, I feel that the cables and appliances on the 240v side of the inverter are adequately protected.

Also, there is the RCD to prevent leaks which would be dangerous to human beings.


Going through this logic again, it seems correct to me right now. But once again, am I missing something?

 

[Richard Burns]

You can obviously choose what you want to do, if you feel that the system is adequately protected then carry on.
Make sure your RCD is working correctly and that you have considered cables /connections overheating in case of a long term small overload and you are sorted.