TEST FOR LACK OF POTENTIAL

[friebel]

I am an instructor at Delaware Technical & Community College, teaching three subjects to electricians from the residential, commercial and industry areas.
At present I am teaching three subjects, and they are,(1)Allen-Bradley SLC500 PLC's, (2)HVAC/R Electrical Control Circuits, and (3)NEC for electricians taking the exam for their Masters, etc.
The very important Safety message that I want to pass on to all electricians out there is, PLEASE USE YOUR ELECTRICAL TESTER to determine lack of potential when working on any circuit, and lock out.
I have been told of several incidents where my experienced electrical students have found switches where the Line side voltage has been put on the Load side terminals.
I know that you realize that your fuses will remain hot with this situation.
We know one reason that this was done. The apprentice electrician had cut the Line-side wiring to short, and they would not reach the Line-side terminals, but they would reach the Load side terminals. The apprentice electrician did not want to tell his boss the mistake that he made. Thank goodness that an electrician that had to work on that circuit, use his voltage tester, and he discovered the load-side terminals still energized when he open the disconnect switch.
It really troubles me when I hear of problems like this, but the bottom line is this: ALWAYS USE YOUR VOLTAGE TESTER TO MAKE SURE THAT THE CIRCUIT IS DE-ENERGIZED.

 

[charlie tuna]

Re: TEST FOR LACK OF POTENTIAL

also consider a square-d design where the disconnect's operational shaft is made of plastic! we found a 480 volt fused disconnect switch that only opened one of the contact blades when operated. sure it sounded great! due to a poor conection on the line side of the switch it created enough heat to actually melt this plastic shaft in half ---- this allowed "b" and "c" phases to remain energized with the switch in the "off" position! always use your tester and just as important ---- confirm that your tester is operational!!!!

 

[dillon3c]

Re: TEST FOR LACK OF POTENTIAL

Originally posted by charlie tuna:
also consider a square-d design where the disconnect's operational shaft is made of plastic!

I don't recall ever seeing one of these disconnects of design like this.Certainly not of the intended "heavy-duty" design..Can you elaborate more, into this Square D disconnect switch?

 

[friebel]

Re: TEST FOR LACK OF POTENTIAL

To: charlie tuna and all electricians. He brought up a good point that I did not cover, and that is to make sure that your testing is functiong properly before you assume that the voltage is not there. Our procedure was, to check it in a 120-volt receptacle to make sure that the tester was functioning properly. Thank-you Charlie tuna for bringing that to our attention.

 

[don_resqcapt19]

Re: TEST FOR LACK OF POTENTIAL

Also be very careful if you ever find a molded case breaker with a broken handle and metal exposed. On a number of breakers this metal is connected to the line side B phase conductor and will be energized even when the breaker is in the off position.
Don

 

[H.L.]

Re: TEST FOR LACK OF POTENTIAL

Originally posted by don_resqcapt19:
Also be very careful if you ever find a molded case breaker with a broken handle and metal exposed. On a number of breakers this metal is connected to the line side B phase conductor and will be energized even when the breaker is in the off position.
Don

I have seen this on 1p breakers as well.
H.L. IBEW LU 134

 

[charlie tuna]

Re: TEST FOR LACK OF POTENTIAL

dillon,
the square-d fused disconnect switch i was talking about "is" heavy duty. it was a three pole - one hundred amp - 600 volt - fused - heavy duty switch. this installation had six switches controlling a/c units. i believe they had number eight wire fused at 50 amps - 480 volts. the construction of the operating mechanism is "totally" plastic. the "b" phase line side contact got so hot - it melted the operating shaft between the "a" and "b" phases. when you turned the switch "off", it sounded normal! except both "b" and "c" phases were still energized --- 480 volts!!! the customer was informed and approved it's replacement as soon as possible --- but i had my men tag both the switch and the unit as a safety measure. we then inspected the other switches and found a second switch beginning to show signs of overheating which we replaced as well. i still have the switch and if you post your e-mail address i can send a picture of it to you. i always thought it would be a good training aid for an apprenticship training course!!! and guys, know that there are many disconnects in the field that have not been operated for fourty years!!!! just because the handle is "off" means zip!!!! use your tester. most everyone here carries a pocket tick tracer these days -- use it!!! and when you close one of these old disconnects --- it might not fully close --- you may now have only 50 per cent engagement of the blades --- so that three hundred amp fuse will cook those blades in a hurry! check for full engaement-- you might have to operate it a few times or lubricate the blade area!!!

 

[dillon3c]

Re: TEST FOR LACK OF POTENTIAL

thanx charlie..Yes I like to see pix.Address on the way..

 

[dillon3c]

Re: TEST FOR LACK OF POTENTIAL

Charlie T,
-yeh- And now.. I'm going to feel,and appear really (idiot-type) stupid/ embarrassed in saying this..So here it comes.. I had know idea,of this just being of the cheap-type Bull___ plastic material,in this operating mechanism alone.Guess it would not have dawned on me or open my eyes then, without the seeing this picture..One picture, forum should see.It could change your way of thinking about safety..It will stay with me,being I easily overlooked it..It was a attention getter for me.

Yes thank you, Charlie T.

[ June 20, 2005, 04:45 PM: Message edited by: dillon3c ]