24vdc machine wiring question

[markymark31]

Hi All !

We have been supplied with a machine that has a 24vdc power cable running through it. It looks like a 2.5mm cable. This is backed up by a 20A fuse in the machine control panel. At various points along its length there is a moulded "junction" where a much smaller cable (maybe 0.25mm ?) "tee's" off this main cable to plug in to some powered conveyor rollers.

My question is this ....

How many regulations is this breaking ?

Now, the rollers are internally fused, so we are only talking about protecting the 0.25mm cable ? A 20A fuse will obviously not provide adequate protection for this cable but it is very short and the risk of damage on this internally wired machine is negligable.

Any advice will be most appreciated.

Regards,

Mark.

 

[Darkwood]

How is the dc created is it through a basic transformer with rectification or do you have a d.c. power supply fitted as these tend to monitor for overload and short circuit protection etc and with respect the extra low voltages your dealing with what are the risks if any if the cable did get damaged?

In respect if you spur off a ring main in a house then this cable is to the spur is in effect over-rated by the 32amp front end fuse and this does leave potential for overload although 2 x13amp load would need to be plugged on it for a long period to effect this cable.

 

[markymark31]

Hi darkwood,

The dc is from a power supply, but I have read other posts that say that these things are not that good with short circuit faults as they may just reduce the voltage to limit the current. With it only being a short length of cable, resistance would be at a minimum. So I = V / R which could melt things couldn't it ?

Regards,

Mark.

 

[Darkwood]

I assume if it did short circuit even if it reduced voltage then the fact the machine had stalled might interupt the control circuit anyway cutting the power but without the wiring plans its hard to say, i really dont consider this an issue due to the nature of the voltage and control system, and dont be mislead by the small cable they can be deceptive in their size if the insulation is only rated for extra low voltage, where the supply ribbon will be 240rated as it runs along side other voltage at some point in its length.

 

[TF and the wire]

I agree with the original poster. There may be current limit within the power supply, but 0.25mm cable is rated around 1.4A so even if the voltage and current cut back in the event of a short, it could still be high enough to melt the cable.

I have seen this happen with a proxy fed via a 3 core 0.25 cable that sustaied damage causing a short circuit current to melt all its insulation.

If I was designing a control circuit, I would feed all proxys, photo-cells etc via a 2A fuse or breaker.