Working near energized 4160 insulated electrical cable

[KSLiller]

We are in need of repairing a leaking roof that has energized 4160 insulated conductors in the vicinity. Do I need to de-energize or can wrap in rubber blankets and would be considered isolated.

 

[powerpete69]

I'm not sure you are going to find a code reference for this.
However, you are talking medium voltage cables and leaking wet roof. (medium voltage to the lay person means super HIGH VOLTAGE)
If the wires are on the older side, the insulation may not be as good as it once was. Hate to see someone get fried on a roof because they didn't de-energize.
I would push to have the electricity turned off. Perhaps do the work on a weekend.
It would be nice if you had a picture of the wires in relation to the roof.
Rubber blanket may work, but are you authorized to be touching 4160V insulated wires? Prob not.
My two cents.

 

[KSLiller]

I'm not sure you are going to find a code reference for this.
However, you are talking medium voltage cables and leaking wet roof. (medium voltage to the lay person means super HIGH VOLTAGE)
If the wires are on the older side, the insulation may not be as good as it once was. Hate to see someone get fried on a roof because they didn't de-energize.
I would push to have the electricity turned off. Perhaps do the work on a weekend.
It would be nice if you had a picture of the wires in relation to the roof.
Rubber blanket may work, but are you authorized to be touching 4160V insulated wires? Prob not.
My two cents.

The cables come into the side of the building and are slightly higher than the roof. We are looking at disconnecting however it is not something that
is easily done as this is a 24-7 chemical plant. The contractors will not come in contact with the cables however they will be working in the vicinity. I am thinking I can put rubber blanketing in the exposed area and should give protection from someone accidentally coming in contact with the cables.
Just looking for opinions on this.

 

[roger]

I agree with Pete, schedule a shutdown. MV is unforgiving.

Roger

 

[kwired]

Are they "insulated" or "covered"?

Not that you still shouldn't have a little concern if "insulated" but if just "covered" you should treat them as though they were bare conductors when it comes to potential casual contact.

 

[powerpete69]

I've seen guys de-energize 480V fully insulated armored cables before touching them to be on the safe side.
Keep in mind, that due to the V^2/R = power principal, that 4160V has 75 times the electrocution power as 480V per the same resistance.
75 times the shocking power of 480V. Just wanted to state that twice.
Since you are a roofer by trade, I would refer to the company's head electrician or engineer and their company policies. Perhaps have their head electrician look at it with you and he can decide if he puts the rubber blankets over them or de-energizes.

 

[Hv&Lv]

Roofers can get a little rough with some things. If it’s metal roof I wouldn’t even think twice about a shutdown. Metal touching the lines with a little for a can cut into the insulation. I agree with some here. Schedule a shutdown, at least when they have to work close to that area

 

[powerpete69]

A little more perspective just for fun.
You've certainly been shocked by 120V. Kind of hurts, right?
4160V has 1200 times the electrocution power of 120 Volts!!!

 

[hillbilly1]

If the lines are owned by the poco, they are usually pretty good at coming out and installing temporary sleeves on the lines if requested.

 

[hillbilly1]

A little more perspective just for fun.
You've certainly been shocked by 120V. Kind of hurts, right?
4160V has 1200 times the electrocution power of 120 Volts!!!

Might not even have to pay for the cremation! LOL!

 

[petersonra]

We are in need of repairing a leaking roof that has energized 4160 insulated conductors in the vicinity. Do I need to de-energize or can wrap in rubber blankets and would be considered isolated.

I would vote to de-energize. You never know just how damaged the conductors might be. And even getting close enough to try and blanket them involves assuming a lot of risk.

Look at it this way. Suppose you go to work up there and something really bad happens and you end up with an unscheduled shutdown of the plant.

Having said all that, a lot depends on just what work you are planning to do and how close the workers might need to get to the conductors.

A serious risk assessment might be in order.

It also might be possible to run some temporary wiring around the conductors in question so the rest of the plant is unaffected.

 

[winnie]

My vote is similar to powerpete69; have the situation evaluated by someone with the appropriate skills to make the situations safe.

This might be a shutdown. This might be sleeving. This might be installing a temporary fence on the parapet. This might be evaluating the actual approach distance and simply marking approach boundaries.

-Jon

 

[Hv&Lv]

I have witnessed a man laying on a wire going through a recloser cycle on a 13.2KV line, and he was experienced.
luckily he lived and is still here giving me grief sometimes...

evaluate it carefully, and don’t be wrong.

 

[Eddie702]

I was working on a job years ago which was a remote boiler room for a prison. Thew were going to put a new service in the building so the old service was dead. They ran a temp feed to an existing panel for lights and so they could keep one boiler running 480 volt. They snaked SER over the top of the bar joists in the roof deck corrigation.

They had a guy on the roof cutting the roof deck with a sawzall to drop a tank in. he cut through the cable. it sounded like a bomb went off. I wouldn't go near 4160

 

[drcampbell]

I disagree. De-energizing is not enough.
Unless there's enough physical separation so there's NO chance of contact -- and you wouldn't be asking if that were the case -- the line should be de-energized, locked out & tagged out, tested by a lineman qualified to work at that voltage and wearing the appropriate personal-protective equipment, then clamped with shorting cables to assure the roofers' safety even if someone does inadvertently turn it back on.

 

[powerpete69]

Just a small chart to add onto multiples of electrocution power as compared to 120 Volts per the same resistance. Basically referring to the watts thru the load....your body. Basically take any voltage, divide by 120V, then square that result. Example, 480/120=4. Then 4*4=16 times the wattage per same resistance as 120V. Again, based off of V^2/R=Power (watts)

208V has 3 times the electrocution power as 120 V per same resistance.
240V has 4 times the electrocution power as 120 V per same resistance.
277V has 5.3 times the electrocution power as 120 V per same resistance.
I think the three voltages above can be the most dangerous simply because people seem to treat these voltages as harmless. You hear of a lot of permanent damage at 277V specifically because it can easily be confused with 120V. Next time you are looking at that "harmless" 240/120 panel, you might think twice as that 240V can really light you up!! 4 times the juice of 120V!!

480V has 16 times the electrocution power as 120 V per same resistance.
2400V has 40 times the electrocution power as 120 V per same resistance.
4160V has 1201 times the electrocution power as 120 V per same resistance.
6600V has 3025 times the electrocution power as 120 V per same resistance.
12.47KV has 10,798 times the electrocution power as 120 V per same resistance.

Be careful out there. And then there is arc flash.....whole other topic....

 

[drcampbell]

... 208V has 3 times the electrocution power as 120 V per same resistance.
240V has 4 times the electrocution power as 120 V ....
And then there is arc flash.....whole other topic.

This is misleading. "same resistance" is just not applicable.
When current starts flowing through a body, it punches holes in cell walls. ("electroporation")
When that happens, an avalanche of decreasing resistance and increasing current begins.
"3x", "4x", and "1201x", et alia, greatly underestimate what will really happen.

With enough time & current, water boils away and flesh chars & smokes. The smoke, composed of carboniferous particles, is much more conductive than air. With enough voltage, current will flash over through the smoke path and then -- as if it hasn't already -- all hell will break loose.

 

[rambojoe]

I could of sworn there is working clearance tables in the nec for transmission lines...but it may be a old field manual, kind of a uglies, but also includes rigging rules and the such.

 

[kwired]

This is misleading. "same resistance" is just not applicable.
When current starts flowing through a body, it punches holes in cell walls. ("electroporation")
When that happens, an avalanche of decreasing resistance and increasing current begins.
"3x", "4x", and "1201x", et alia, greatly underestimate what will really happen.

With enough time & current, water boils away and flesh chars & smokes. The smoke, composed of carboniferous particles, is much more conductive than air. With enough voltage, current will flash over through the smoke path and then -- as if it hasn't already -- all hell will break loose.

I have to agree with you. "electrocution power" is misleading. What he did describe was difference in energy for different voltages with same circuit resistance.

It only takes a very few milliamps at about any voltage to stop the heart, but higher the voltage is the more chance it may have at enough current finding the right path. Can still have a lot of cell damage in non fatal incidents as well. In general under 50 volts is not much threat, but can be if conditions are right.

 

[drcampbell]

Electroporation often causes the victim to drop dead on the fourth day after appearing to recover from the initial electrical-contact trauma. Holes in the cell walls make it impossible to maintain electrolyte balance.