PLCS.net - Interactive Q & A

PLCS.net - Interactive Q & A (http://www.plctalk.net/qanda/index.php)
-   LIVE PLC Questions And Answers (http://www.plctalk.net/qanda/forumdisplay.php?f=2)
-   -   Light guards: PNP vs NPN & safety (http://www.plctalk.net/qanda/showthread.php?t=21837)

Steve Kemp April 6th, 2006 08:32 AM

Light guards: PNP vs NPN & safety
 
Hi all. It would appear that the japanese "standard" is to use npn devices. I've always used pnp and grounded the 0V of the DC psu at source. However, if these guys are used to everything being this way up, is there a problem with my tying the +24VDC rail to the machine's chassis? I can't think of one and I'd sooner do that than leave it all floating. Any thoughts? (now that's a silly question).

CaseyK April 6th, 2006 09:38 AM

I like to ground the "0" line myself.

Now, if you are building something for "them", then you may want to used "their" standard.

If it is something for you, then that is another choice.

I have seen a couple of machines that had both positive and negative logic being used, so no grounding was used. The grounding became an issue with an local building inspector. He could not provide a solution, so it went ungrounded. City officials later asked why he was poking ariund in a machine, anyway.

If you do ground the positive, you will not be xast out of the plcs.net family.

I did a poll on this about a year ago. "POLL"

best regards.....kc

bob1371 April 6th, 2006 09:54 AM

Steve,
http://www.mrplc.com/kb/index.php?pa..._v2&id=44&c=38
Here is a great link that was posted in this thread.
http://www.plctalk.net/qanda/showthread.php?t=20812


Bob

Steve Kemp April 6th, 2006 10:01 AM

Bob, I've had a print of Jim Rowell's article in my desk for some time now; yes, it's a good reminder.

panic mode April 6th, 2006 05:21 PM

we do a lot of work with npn guys (japanese car makers).
positive side of 24V supply for I/O is grounded. safety circuit is on separate supply and that supply has negative side grounded. i've never seen it but i was told that in japan one can buy safety products like light curtains with pair of npn outputs.

Steve Kemp April 7th, 2006 02:14 AM

Mr. Mode, you're right. We're using the SL-C range of light guards by Keyence. Both o/p types are present at the receiver connector, making it universal. Buying the appropriate cable (n or p type) connects the same colour cores to the npn or pnp ossd's -neat.

...And, reading the Standards and Regulations section in the SL-C glossy, I note that this model -even when used in conjuction with the dedicated controller- has not received certification in accordance with Article 44-2 of the Japanese H & S Law, pertaining to its use with presses and shearing machines. I find this a curious irony, given that our customer imports almost all of its test/assy equipment from Japan (CE marked), using light guards to inhibit a minimum of outputs -in one case, just the downstroke of a pressing cylinder- via the (non-safety) plc. There are no pneumatic dump valves, no isolator taps, and gaps in the "guarding" through which various limbs or small humanoids can easily pass. Funny ole world.

ushidayo April 7th, 2006 02:56 AM

All the systems I've seen in Japan use NPN sensors and the DC24V is left floating. Machine safety doesn't seem to be such a high priority here as it is in the west. No lockable isolators and e-stops connected directly to PLC inputs. Maybe the operators are a bit more careful here, or perhaps they just keep quiet when they lose a finger!

panic mode May 1st, 2006 10:17 PM

Hi Steve,

You are right. It's quite strange but that's how it is. At least we can joke about it. I checked SUNX catalogue (customer wants it) and they also have light curtains with NPN outputs. And there was same note about such products being inadequate for press applications in Japan. Also there is statement saying that they never asked for evaluation by 3rd party / independent recognized institution, they just did their own test and found that is is ok. I called supplier and asked about it, since customer wants to use PNP version of same product on a press.... They did send me somehting that looks like proper certificate (this was probably done after catalogue was printed), but I would rather see it in the catalogue or datasheet of the product than on a piece of paper comming out of fax machine...:huh: :boo: :smas: :shutit:

OkiePC May 1st, 2006 10:34 PM

Am I missing something?
 
The important part is not so much the use of NPN outputs as which side of the PS is grounded. You don't want a short to ground to ever be able to cause inputs to go true. Especially inputs to a saefty relay! They are grounding the positive right? So, the negative still has potential to ground and will (hopefully) trigger a short circuit protection device if it is shorted to ground, and even if it doesn't, can't cause the relay to pick up.


Or, am I missing something? I have been confused on the subject before...

I had a bunch of machines with sourcing inputs from photoeyes and a negative (or 0vdc) ground. Any time one or more of the inputs were "stuck on" we knew there was likely a short somewhere. It wasn't dangerous, but could cause waste and confusion. When we upgraded from a mechanical drum stepper to SLCs, I switched them to sinking inputs and it was much better, now the machine just stops, creates no waste (now a short to ground does not trigger a change of states and advance the cycle), and is easy to troubleshoot.

Quote:

...Imagine my surprise when I noticed the inputs to the PLC were all from the grounded side of the power supply....
by Jim Rowell


http://www.mrplc.com/kb/index.php?pa..._v2&id=44&c=38

Okie

EDIT:
OH, AND MY HAT'S OFF TO THE AUTHOR! THIS IS THE BEST INDIVIDUAL COMMENTARY ON WHAT NOT TO DO WITH CONTROL SYSTEMS I'VE EVER READ. I FORWARDED A LINK TO IT TO OUR ENG. DEPT. HOPEFULLY THEY WILL READ THE WHOLE THING AND TAKE IT TO HEART.

MANY THANKS
PAUL


All times are GMT -5. The time now is 07:59 PM.

.