Circuit breaker and fuses

Hey Everyone,

Hopefully this is a relatively simple question.

Firstly, it may be important to know that I'm building a motion control video rig, not a CNC machine.

Inside my control box I'll have some drivers of very different sizes. As such, it was suggested to me to use circuit breaker / MCBs to ensure I protect the smaller drivers. My original plan was to have an IEC socket with a switch and fuse but now I need to figure out what MCBs to get. Apologies but I'm still quite new to this.

These are the larger drivers:

2 phase large driver 1 x DM2282T

3 phase large driver 1 x DM2283T

They have built in PSU's. I'll then have 4 smaller drivers:

4 x DM542T

They will be powered by this 350W 48V PSU

Two questions:

1) Power is coming to the control box from the mains and plugging into an IEC socket which has a fuse. Should I still use that fuse if I'm adding MCBs?
2) What size MCBs do I need?

Thanks for any help.

Simple question, but the answer's a bit more complicated...

First of all, fuses/MCBs are not really there to "protect" devices downstream. They might, under certain circumstances, but in general overcurrent devices like fuses are to protect against the risk of fire in the wiring when there is already a fault downstream of them. For example, motor wiring is shorted where it runs round a sharp corner; the driver (if it does not have effective overcurrent control) pops its output transistors, too much current drawn from the supply, wiring overheats - then fuse blows before it gets out of hand.

On the plus side, the PSU to which you have linked has built-in overcurrent/short circuit protection - it says. That might be enough to protect against any downstream faults of any significance.

On the down side, a switch-mode PSU is not ideal for stepper drivers Iwhatever the ebay ad says). Stepper drivers take a modest average current but it tends to be very "peaky" in that the steppers are driven with a series of high-current pulses which average to a modest value. SMPS do not like high-current pulse loads and tend to go into "protection" mode if they see them. You have to stay well within the ratings to avoid this, remembering that your drivers will all be demanding pulse currents at different but sometimes overlapping moments. It's not easy to do the sums, which is why most CNC builders (and I note that this is not a cutting machine that you are building!) use "linear" power supplies - wihich are bigger, heavier, a bit more costly, but easy to build and relatively bullet-proof.

How big an MCB? Your power supply is rated at 350W output. Let's be conservative and say it needs 500W input (it will be less in practice). That's only 2A from a 240V supply (will, near enough). Maybe a bit more to cope with switch-on surge (which is a problem with linear supplies but not much with a SMPS). So, 5A MCB? And make sure that the wiring from MCB to power supply, etc is rated at at least that - not difficult - and you will be fine.

Others may, and almost certainly will, disagree...

Thanks Neale! Very helpful reply which sadly has left me with a few more questions.

1) If the PSU has overcurrent / short circuit protection do I even need an MCB? It sounded from your explanation like the MCBs wouldn't be much help anyway. It also seems as though the SMPS is the wrong choice but I'll probably just switch it out in V2 of this build, I need to get this build completed ASAP.
2) So a 5a MCB would be enough for the PSU and smaller drivers but what about the larger drivers?
3) Would it be a bad idea to have a fuse at the IEC socket combined with an MCB?

Thanks again for the advice

A fuse and an MCB will not do any harm - leave the fuse in place if that's easier. I have an MCB in my own control box - first thing the mains gets to, before anything else, so that virtually everything in the box is then "downstream" of the MCB. Didn't help when I refitted something and put a bolt straight through the insulation of the incoming mains cable without noticing - tripped the ring main MCB (32A) plus consumer unit RCCD (60A, I think). Didn't blow the 13A fuse in the control box mains plug, or the one in the extension lead plug. That might tell you something about MCB versus fuse performance!

Don't care if the motors take 10A, say (if the PSU could deliver it). That's at a low voltage. The problem here is that the incoming mains is at a much higher voltage. For the same power, the higher the voltage the lower the current - power = volts x amps, in simple terms. So if you take 10A at 36V from the PSU, that's 360W (watts). At 240V, that's 1.5A (=360/240). I'm simplifying slightly here but that's pretty much correct.

Don't care about driver ratings, big or small. It's what they actually deliver to the motors that matters; Stepper drivers usually have a "max current" adjustment, so a "big" driver could be set to 2A while a "small" driver could be set to 4A, perhaps; Daft way round to configure them, but it's possible. If they both use 36V, then the power delivered (which comes from the PSU) will be 36x4 = 144W or 36 x 2 = 72W. So, forgetting any power loss in the PSU (which will be small) then this is the power taken from the mains. But that is delivered at 240V. Current draw of 144W at 240V = 144/240 = 0.6A or so (I'm doing the sums in my head).

To calculate the MCB size needed you have to estimate the current going through it; start with the power that the load takes (volts x amps). then convert this into current at 240V (power / 240). You are limited by whatever your PSU is rated at - 350W, it says, and whatever you try to draw from it, the PSU will limit to this value - so at mains volts that's 350//240 = 1.5A (near enough). You can connect to 100 stepper drivers of whatever rating you like - but that PSU will still limit you to a total of 350W (7.3A at 36V, like the ad says) in any case.

What Neale said... at least peak currents, but the stepper drivers will be generating a low voltage, high current drive to the motor - substantially less voltage than the terminal voltage on the stepper driver. So the *average* power consumed will be significantly less than the worst case above. Amazingly, those linked datasheets don't describe the input power requirement - go figure! At least the DM2282 has an indicative peak current specification on the case - I'd add that to a notionally similar one for the 2283 and 2A for the SMPS, add a fiddle factor of 1.5 to the total and come back and complain when it doesn't work.

Each device is likely to take a surge current on power-up, likely higher than the operating current.

Not sure that surge is as much a problem with SMPS as with a linear supply as the storage capacitors are much smaller, but there will be some. Frankly, it's all a bit finger in the air but for what it's worth, I have a machine with 4 steppers. drivers set to deliver around 4A each at a nominal 70V. That's about 560W as I assume that only two steppers are taking full current at any one time. As I say, there's a fair bit of guesswork in all this! The rest of the control box (motion controller, relays, etc) take so little that they aren't worth considering. However, I have fitted a 10A D curve MCB. 10A gives headroom to allow for switch-on surge for the linear supply, and the D curve is there as it has a slower response time to help with this.I am confident that even with a major fault internally the MCB will trip before the wiring bursts into flames - and, frankly, that's the only protection that the MCB gives.

One MCB for the whole box - I don't feel that there is much point in a separate MCB per driver as some people do. At least my way if the MCB trips then everything is going to stop rather than just one axis. But.- you pays your money and you takes your choice.

Hi both,

Thanks for the responses. I'm not going to pretend that I'm not a little confused, been so out of my depth on this project, so please bare with me.

For the SMPS which drives the 4 DM542Ts, the drivers will most likely be running at 1.36 amps (RMS), not really sure if I'm supposed to go by RMS or peak but that's another question. I read somewhere that if micro stepping you should go by RMS?

So (theoretically) we have 4 drivers running at 1.36 amps = 5.44 amps

Now I just need to work out the volts. Correct me if I'm wrong but it would not be a good idea to run at 48V on the PSU? Someone mentioned this to me before as it's very close to the 50V limit of the drivers. As such, what would be a safe voltage to run the PSU at? Once I have that number I can do the calculation for power and work out the MCB for these drivers.

Once I've got this correct, I'll move onto the bigger drivers.

Don't overthink this one - concentrate on the important bits and the rest will fall into place or is just not that critical. Voltage is important. I would aim at a nominal supply voltage around 10-15% lower than the max driver voltage. For example, with my 80V drivers I run them from a nominal 68V supply. However, if you are using SMPS supplies then you are a bit restricted by what's available so a 36V supply would be safe for your 50V drivers where a 48V supply would be bordering disaster.

After that, think about current. This whole "is it RMS or peak?" question is always coming up, and I'm not even sure which mine mean when I am setting max current. Again, rule of thumb is that if the motors don't stall then there's enough current; if they get too hot then there's too much current. "Too hot" for a stepper means that it could be just a touch above "too hot to hold" but not much more. Assuming adequate ventilation, anyway.

So, what PSU rating do you then need? With a linear supply there is much more leeway as they are very happy with the kind of short-term pulse load from steppers as long as they can reasonably cope with the average. Because of the way they work, SMPS supplies cannot tolerate even short pulse overloads so need to be sized accordingly. Fortunately, it generally doesn't cost that much to go up a current rating or two if you are buying from China so you can err on the safe, over-specified, side. If you are likely to have multiple motions at once (seems plausible with your system) then just add the driver max current settings you are using and use that as the PSU rating, rounded up to make sure there is some headroom.

For the MCBs - I would expect that 5A would be enough for your system. That's a fair bit, but you could go up to 10A if not (and they aren't that expensive). Frankly, there isn't that great a choice of current ratings so it's not as if you can fine-tune anything in this area.

Keep the questions coming - I think that all the regulars on this site would rather answer a question than see someone get it wrong! And don't just listen to me - my views are not always completely in line with other people. I was trained as an electronic engineer but one thing you learn is that theory might suggest a good way to go, but you have to take practical aspects into account as well.

Sorry for the brief response, I'm out of the house but wanted to reply. I'll reply properly tomorrow.

Firstly, thanks again. I really appreciate the time you've taken to respond.

My PSU is 48v but I believe I can adjust it. Would that be ok? If I adjust it and measure it with a volt meter to around 36v? Or is it better to purchase a different PSU?

If it's adjustable then just drop the output voltage to 42-43V or so - that would be about right. You are aiming at highest voltage compatible with the drivers' ratings.

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

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.

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

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.