sinking start commands illegal??

I've just had a new conveyer weighing system installed that I have discovered has neutral activation. I was always under the impression that switching functions should always be wired asasourcing inputs only. So trying to be diligent I started reading the Canadian electrical code. The only excerpt I could find was rule 14-416

"Control devices used only for switching. Except as permitted by other rules in this code. control devices that perform only switching
functions shall disconnect all ungrounded conductors of the controlled circuit when in the off position."

My question is does this regulation apply to my concern? If not does someone know where I can find the regulations that I would need in order to get the contractor to change it to source activation.

Nuetral and DC common are not the same thing. If you are talking about DC circuits and sinking outputs, positive common would be a better term.

That sinking feeling

This section of the code refers to the switch connected to a load.
14-400 says " control devices shall have ratings suitable for the connected load...." The intent of this section was to make sure "control devices" do not open the neutral conductor, but open the "hot" conductor making the circuit dead or "safe". This doesnt apply to PLC control. The outputs on the cards are always "Hot" from backfeeding. Unless you are using relay output cards. Input cards do not control anything directly. They only turn on an bit in computer memory. To satisfy this rule control consoles have a Control power pushbutton that is maintain open or closed, this removes all power to the PLC rack via a relay or whatever. Some companies interpret this as the removal of all power to all the I/O on the PLC in question. There are some very creative ways to implement this when you have 12 racks spread over 100 meters of mill floor, on 3 different floors.
There are some good books on wiring PLC contol. The Allen Bradley
Installation and maintenance guide is OK to start off. You can order it off the web site.

Bruce

This is unrelated to the Canadian electrical codes but..

If you connect a "start" pushbutton to a "source" input and the DC common is grounded then there will always be a risk that a false start signal will be received (if the conductor between the pushbutton and the PLC input is accidently grounded).

As a rule of thumb, only use sourcing input modules with an ungrounded DC common.

Thats how I always understood as the safest method of hookups on the start commands or any inputs that activate any moving parts. The code references I need is to allow me to convince the designers of said equipment. As it turns out the equipment in question has been installed in numerous plants. So its not a simple matter of merely correcting the equipment installed in just this location.

I have no idea about which Canadian Electrical Code might contain the reference you want. Are there any regulations in Canada relating to machine safety? In particular, unexpected start-up of a machine? It may be worth trying a different track, other than electrical regulations, i.e. Health and Safety.

Unfortunately, the conveyor is already installed and your supplier probably just wants payment and a swift exit.

Whatever happens, the design fault has been identified and requires resolving (if someone is injured, the consequences for your company aren't good...). Have you involved whoever is responsible for Health & Safety in your company?

Good Luck

P.S. Here in Japan, I see source input modules all the time but this is OK because the DC common is not grounded.

sinking ..........

Hey Mordred,
Did a quick look around for you and again, the code is not really related to the PLC. I did a quick check here, we use both sinking sourcing and ttl for "starting". The important thing here is power supply isolation. The common must not be grounded. If the module and power supply are wired correctly, accidental grounds on control circuits will do nothing. The code deals with real loads and not computer bits. We do not want to open the grounded conductor on light switch or the heater, most people "think its off" and therefore safe. Open the neutral and this is not the case. Sinking inputs are not the same. I am not a code expert, but I have been in the safety part of industry for a long time. The "what ifs" are what safety is all about. Personaly I dont think I would use DC as a control voltage. Some of our contractors use flex I/O at 24 VDC. The engineer on the design team will deside for me what to use. I just have to make it work. heh heh heh.....
Sorry, I have to get to bed... G'nite
Have a good week end.
Bruce

I found the code under 14-016 of the CEC. they had wired the start commands to the grounded conductor with the neutrals connected. From what you stated this should work provided that we disconnect the 0 volts from the ground. If thats the case then the matter becomes a ather easy fix. The engineering team are currently looking into the matter. Now it becomes their decision on how to proceed.