Cutting 24vdc wires

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cornbread

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We had a contractor doing some demo in a marshalling panel to remove some 24vdc wiring. The circuits were identified and knife switches opened to de-energize the circuit. Making a long story short they got on the wrong wire (still hot 24VDC) and when they cut it, it shorted out the 24vdc power supply dropping the process. Granted he should have checked the wire before cutting it, but we were discussing the proper way to cut wires. With any twisted pair cable I’ve always cut one wire at a time never cutting thru the shield and both positive and negative at the same time. Basically I got your and idiot look from him. Is it good practice to cut wires one at time or am I way off base here. Your opinions as always are greatly appreciated.
 
There is no way I am going to start cutting shielded control cable conductors one at a time as a regular way of working.

That would be reserved for the oddball times where identification is in question.
 
I agree with iwire.

Also why didn't the individual cables have their own OCPD so you wouldn't take out the power supply on a short circuit?
 
cornbread said:
We had a contractor doing some demo in a marshalling panel to remove some 24vdc wiring. The circuits were identified and knife switches opened to de-energize the circuit. Making a long story short they got on the wrong wire (still hot 24VDC) and when they cut it, it shorted out the 24vdc power supply dropping the process. Granted he should have checked the wire before cutting it, but we were discussing the proper way to cut wires. With any twisted pair cable I’ve always cut one wire at a time never cutting thru the shield and both positive and negative at the same time. Basically I got your and idiot look from him. Is it good practice to cut wires one at time or am I way off base here. Your opinions as always are greatly appreciated.
Click to expand...

the largest single power outage in LADWP history, occurred from doing exactly
what you described was done here.

http://www.washingtonpost.com/wp-dyn/content/article/2005/09/12/AR2005091201791.html

there was a 5 conductor control cable coming from the switchrack at RS-D on venice blvd.
an idiot from the same bin as your idiot cut across all of them with his dykes, and put 250 VDC
control power across a SCADA bus, god knows how. the lights in the control house blinked twice,
then went off, along with most of LA.

the 230 KV bus relayed open all over LA. they had to manually open everything, and bring the
bus back online, then reconnect all the positions manually. sort of a cold reboot of LA. station operators
had to physically drive to substations in gridlocked traffic 'cause there were no traffic signals, open positions,
then return later to reclose them. most substations in dwp land are unmanned.

there was also a phase shift at RS-D, causing somewhere between $40~$50 million in hardware
damage, in addition to liquidated damages caused to customers of LADWP.

the fine fellow who did this, did so after being directly told NOT to cut control cables in that fashion.
as he was a civil servant, who cannot be fired, i suspect his career path did change direction however.
probably a nice window seat at the barstow solar array, inspecting the path of the sun to make sure it
stays on track.
 
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