Testing Error

Dear all PLC experts,

I'm a fresh graduated junior electrical engineer now involved in a PLC project of an automation machine. I encountered few problems during program test.

I am using a Shilin PLC now. I tried to test my software by using the 24VDC service supply connecting to the input contacts. there was a short circuit that cause the communication cable and computer com port spoilt. The reason of short circuit is because the service supply accidentally touch the metal case of a 24V adapter. The controller was fine fortunately.

1. I could not understand why the front part of the cable and computer comm port spoilt while the end part of the cable and PLC were fine?

2. Testing using the 24V service supply to the input contacts to read the input and output lamps were very dangerous. Two case of damages to device already happened. Is there any better ways of testing the PLC in a safer way?

Please post advices. Thank you.

sutton

What do you mean by spoilt? Arcing, melting?

As for testing your inputs safer, you could wire between the inputs to a terminal block so you can avoid accidently shorting to earth. Or you could purchase some test leads that have only the smallest tip at the end of the insulated probe. Wire your leads to the 24v source and away you go. I bought a full test lead set from Fluke for around $40AUS ($20US) thats comes with alligator clips, the needle probes, and probes with extra long leads. And it even came with its own little zip up carry bag!

Cable

dear TimeFluxCap,

Thank you for your advices on safety testing. while using the way you prompt, I had also consider of using several small selector switches connected the input contacts to the 24V service supply. In that case, I only need to switch on and off the desired inputs instead of testing directly from the 24V service supply. Is that an ideal way?

About the cable, my senior took it away from me to test what goes wrong. He came back and told me it was 'spoilt' and i was scolded for the damage. well, i wonder myself was it melted or arched. but i think if it is either case, could i repair it by simply soldering the damage part? or is there any better way to repair it?

thanks.

Sutton

He might be playing games with you perhaps?
If the lead has continuity on each wire and the plugs are fine there is no problem.

Plug it back in and try to comm with PLC. If it works then all is well for your comm port as well.

PCB in the cable head

TimeFluxCap,

From your explanation I quickly went on to check the cable again. I eventually found out that it's the PCB part in the cable head spoilt. well, I'm now using Shihlin PLC which is similiar to Mitsubishi Electric and run by Melsec Medoc software. They designed they own cable for the use of their own PLC. And that's why they sell it so expensive (cable=US $80). Some part of the PCB was burnt, and the damages to the chip could not be repaired.

However, I still could not understand what cause the damage to the cable during the accident. It was very complicated i could not understand why there are short circuit when the 24V service supply touch the 24V adapter metal case. As far as i concerned, the metal case does not conduct electric from it's internal electronic devices.
It's annoying to me why only the cable PCB was burnt but not other part of PLC.

sutton

Im unfamilair with this type of lead Sutton one of the other guys might be able to help. For it be damaged it must at some stage been part of the short circuit.

I was going to answer this earlier, I assume that since a cable with a PCB board was used then its converting something like RS422/485. In some instances the metal body is connected to the common voltage and to have a source of voltage connect to that point could short items on the PCB.

It is silly to risk damaging an expensive PLC to save a little money on switches. Go to your local Radio Shack or equivalent, buy a few toggle switches and relays, and when you test your logic take the extra time to wire them in properly when the PLC is powered down. After having let the smoke out of a number of devices over the years, I learned that taking shortcuts to save money is incredibly expensive!

RE - testing error

Maybe I'm wrong, but I get the impression that this guy is nowhere near a Radio Shack store. Having said that, I have had some experience with power feeds both in Europe and the far east where the control voltage is 220V and both conductors are hot to ground. Some PLC equipment considers the neutral connection to be grounded by default and connects neutral to ground. With this in mind, the chassis of the PLC (and the connector shell for the programming cable) would become hot to ground. If the assembly was not grounded in any other way and the cable was then connected to a laptop or other equipment in which the _other_ side of the line was connected to ground it would not surprise me if the cable fried. This is not theorizing by the way, it has happened to me on site and this is what the problem was determined to be. Since then, I always bring an isolated power supply for my laptop when I go overseas.