Sorry Jameselectric - but I have to say you may be well in the wrong - or possibley misunderstood your task - you were supposed to make it safe for the plumbers to do thier work. The "how" - is not in the "why" here in your case IMO. You were asked to "Safe Off" +/or LOTO some equipment for some non-electrically enclined Plumbers. You showed up - 'traced out' the circuit shut it off and left a piece of 'tape' over the breaker.... (Then someone turned it back on??? Or otherwise turned into a fiasco...) Does your shop or facility just have a "Personal LOTO Program" and
lack a "Group (Facility) LOTO Program"?????
IMO - you should have gone a few steps further. Traced and double checked that all of what was on the circuit were supposed to be off, and seperated them - tagged the wires and tag in the panel with contact info to reconnect. Or properly locked the breaker with "YOUR" lock and tag with the info, and then went and got a new one for yourself.... Or just waited to get your stuff back.
As it sounds like the plumber is not qualified to re-energize the equipment when he's done with what he was there to do. So having his lock there may have been more of a hazard - which is why you were sent to do it.
Truthfully if your
ONE PERSONAL LOCK is so precious in this instance - Then you're definately in need of a dozen or so (Group) Exended Electrical Outage / Operations Supervisory Locks each with a multi-lock hasp for the Electrical (Craft) side of your facilty... You can order a case or more all master/tier keyed and supervised by your shop. And short the expense of getting a fancy tag for each, you can go to a copy shop and fire off custom ones, at least one for each and get them laminated for ~10-20 bucks, as for the breaker and switch fittings just suck it up and order them. Number them and make a chart for where and why stuff is off, with a key log and set procedure for access. Because you are going to have to leave stuff locked sometimes - that is just the way it goes.... Even if your lock is not there - it still gets locked and stays locked untill someone is assigned from your shop checks out a key, and clears the equipment for service to remove it. All personal LOTO is on top of this...
http://www.osha.gov/dts/osta/lototraining/hottopics/ht-gloto-3-1.html