Powering Down The PLC

[Aabeck]

Just came across a second panel this month where the PLC lost it's program due to a low battery & the power being dropped. On both machines the PLC was (keyword = WAS) powered by MCR power, not line power (I rectified that situation while I replaced the battery & reinstalled the program I had a copy of). I have never done this, as I figure the PLC should be on whenever the service disconnect is on, just use MCR power to supply most outputs, leaving some outputs line powered (Red stacklight, MCR enable relay, Buzzer, etc. but never any process items) with the PLC monitoring if MCR power is on. Also, this PLC had MCR power wired to an input & did check if it was on to run things, but if MCR power was off the PLC would be off.

What is the reason, if one, anyone would wire a PLC to MCR power?

Am I alone in this thought?

 

[jraef]

Lazy? Ignorant? Newbie? Obstinate ("Because I've always done it that way")? Obliviot (oblivious idiot)?

 

[Aabeck]

You forgot "idget" - that's Mental Midget, where the 2 'M's cancel each other.

 

[Mike Lamond]

I once ran into the exact same problem on a small conveyor system. That was caused by "BIL Syndrome" - the control panel was built by the plant manager's brother-in-law!

 

[dploof23]

Obliviot (oblivious idiot)?

I love it.....had not heard that one before.

 

[jraef]

I love it.....had not heard that one before.

I came up with that while driving one day. Oblivious idiots all over the road making really stupid decisions with no clue as to the effects.