Circuit breaker and fuses

[mbridge87]

Finally got some time to reply! Apologies, I was on a shoot yesterday and have a funeral today so things have been a little hectic.

I've re-read this whole thread and understand things a little better. My original intention here was to have two separate MCBs due to the very different capabilities of my drivers. One would be for the two large drivers and one for the PSU + 4 small drivers.

That said, while my steppers motors are quite different, the drivers are going to be set at:

- around 3 amps for the big steppers. Not sure of the voltage, can't see where to find this on the big drivers
- and 1.36 amps (42.5 Volts) for the smaller ones.

Therefore, given the power required for each won't be that different, it doesn't make sense to have separate MCBs. Rather, I should (or could) just have one to protect the whole system against cables setting on fire.

As I plan to one day have all 8 axis being used, I think I'll go for one 10A MCB. This is the one I'm thinking:

https://uk.rs-online.com/web/p/mcbs/8255633/

That all sound good or am I still getting this wrong?

FYI, these are the internal cables I'll be using:

Power - https://www.amazon.co.uk/gp/product/...?ie=UTF8&psc=1
Signal - https://www.amazon.co.uk/gp/product/...?ie=UTF8&psc=1

[Doddy]

Without spending too long looking at this (At work), I’d question the logic of protecting cables rated at 7.5A with an MCB rated at 10A.

You could be thinking that you have all these separate loads and wiring them back to a star point, but if you have a single point failure (most common) and you generate a fault current larger than the rating of the individual circuit wiring is rated to, but less than the mcb, then you’re not protecting that circuit.

Back of my mind there is head room built into all the specifications, but on paper your choice of power cable doesn’t quite match.

The mcb is specified to tolerate the states current, and will trip some way above this (google fussing factor, or something similar) your power cable should be rated accordingly.

Btw, personal thoughts: you’ll get frustrated with 4m reels of cable!, better to get a big reel of a neutral colour and cable markers, or at least larger reels of the main black/red if that’s your colour of choice.

[mbridge87]

Hi Doddy and Neale,

I purchased the 10A MCB which I referenced in my last post. I did this before I saw Doddy's message. Fingers crossed this and the cable I'm using is suitable but I'd love to hear your thoughts Neale if you get a moment. The idea is just to run everything (Live) through this MCB and not have separate MCBs as I originally intended. Everything is explained in my last post.

Cheers, Max

[Neale]

Quick answer - looks fine to me. Theoretically, as Doddy says, the ratings don't quite match up, but things are just not that black and white. From memory, the mains cabling in my control box uses cores stripped from an old appliance cable (junked washing machine or somesuch) so has a higher rating than the cable you are suggesting, but given that your cable has silicone insulation and has a max temperature rating of 200C, I wouldn't worry too much.

MCBs have two tripping mechanisms. One is for major overload and that's mainly where the B/C/D curve business comes in. A D curve MCB will trip at 10-20x rated current in <0.1s - so will trip virtually immediately if there is a major fault. However, they also have a thermal trip - it heats up with increasing current so a smaller overload for a longer time will also trip it. That should roughly match the characteristics of your cable - if overloaded it will start heating but so will the MCB so as long as the MCB trips before the wire overheats excessively, that's fine.