1000 MCM single conductors

Hello, I have to replace some old wooden vertical raceways (out the top of a PDC xfmr room) that have 1000 mcm single conductors, with some steel tray.

Does anyone know of any health hazards to do this while their is large currents flowing?

Thanks, Dave

Reply to
Dave
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Dave

Reply to
Dave

Health hazards? None that I know of. I doubt that the feilds produced would be high enough to offset your cell phone usage or the 2 way radio you might use daily.

Reply to
SQLit

Old, you said? 1000 Kcmil with current in them and you are going to replace the wooden support with tray while they are energized? Yes, I would say there is a health hazard - your life. De-energize the conductors or use proper protective equipment including gloves, face shield, insulated gloves, and have a worker qualified in cpr and who is also equiped with the proper ppe standing by. If you are in the USA I would ask for a voluntary OSHA compliance inspection and a safey plan.

Reply to
Gerald Newton3

Thanks, They are 480 V

Not high voltage.

Dave

Reply to
Dave

I think the point is that conductors of that size might well have substantial SC capacity, which if you managed to nick a wire could result in a very serious arc flash. you should really try to do this with the power OFF.

Reply to
Bob Peterson

That's good advice. I'd also worry about the possibility of damage to lugs, bushings and other hardware to which these cables are connected if their mechanical support is compromised during the work.

Reply to
Paul Hovnanian P.E.

480 volt is deadly when it comes to arc flash. After working for six months at a 24 year old industrial facility that had a lot of distribution at 480 volts, I would not assume any thing as safe anymore. Once I turned on a 480 volt three phase 70 ampere main breaker and got the shock of a life time. The plastic breaker handle cover had gotten broken and only the metal part was left. There was 277 volts to ground on the metal. I was wet and had wet gloves on. At this same facility cables had been stretched very tight in 1982 when a 120 pound per square foot snow load broke down the cable tray support structures leaving the cables taut. The cable were never replaced. You never know what took place in the past. Do not assume that anything is safe in the electrical world!!! While working next to a journeyman with over 30 years experience I witnessed his screw driver exploding while he was in the bucket. The 400 ampere 480 volt breaker that he had turned off was still hot on the load side!!! The breaker was defective. A journeyman I know was working some cables in the back of a distribution center in that was hot. One of the existing cables came out of its terminal and cross phased. It blew him several feet and burned him badly. This stuff is dangerous. I know a person who had a bad experience with arc flash who now never ever stands in front of 480 volt equipment when he turns it on. He turns it on with a stick while he stands to one side. Talk about arc flash shy!
Reply to
Gerald Newton3

Thanks for all the safety advice. The job is complete. I replaced the old wooden raceway with modern electrical tray. I was not really worried about arcs. I was worried about the extended time being in close proximity to any EMF surrounding the cables. I made sure that no forces were put upon the teck connectors. This PDC room was designed 40 + years ago with the secondaries coming straight out the roof of the room. Most PDC rooms these days have the secondaries coming out the side of the building or out the bottom, which makes it much easier to seal around the cables / tray etc. for moisture.

Speaking of sealing, does anyone know of any potting compound one could use in a framework around the tray and cables to seal where they exit the room? We already have foam, but the Engineer wants a potting compound pored in a framework around the cables and tray.

Thanks, Dave

Reply to
Dave

For what purpose? 3M makes fire stop products.

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Tim

Reply to
No Spam

if you are worried about EMF, you are seriously worried about the wrong thing. There is no serious science that suggests short exposure to such levels of EMF you experienced has any negative effects. There is a LOT of experience that says that the work you performed is quite hazardous. the problem is that while the chance of something going wrong on any individual exposure to this type of risk is fairly low, the consequences are very grave, and often fatal. Arc flash is something you should take very seriously, as it has killed and/or seriously injured a number of people over the years. You ought to take some time to read up on this, so you will at least understand how dangerous what you are doing actually is.

Reply to
Bob Peterson

On Wed, 24 Nov 2004 20:13:21 -0900 Gerald Newton3 wrote:

| 480 volt is deadly when it comes to arc flash. After working for six months | at a 24 year old industrial facility that had a lot of distribution at 480 | volts, I would not assume any thing as safe anymore. Once I turned on a | 480 volt three phase 70 ampere main breaker and got the shock of a life | time. The plastic breaker handle cover had gotten broken and only the metal | part was left. There was 277 volts to ground on the metal. I was wet and | had wet gloves on. At this same facility cables had been stretched very | tight in 1982 when a 120 pound per square foot snow load broke down the | cable tray support structures leaving the cables taut. The cable were never | replaced. You never know what took place in the past. Do not assume that | anything is safe in the electrical world!!! While working next to a | journeyman with over 30 years experience I witnessed his screw driver | exploding while he was in the bucket. The 400 ampere 480 volt breaker that | he had turned off was still hot on the load side!!! The breaker was | defective. A journeyman I know was working some cables in the back of a | distribution center in that was hot. One of the existing cables came out of | its terminal and cross phased. It blew him several feet and burned him | badly. This stuff is dangerous. I know a person who had a bad experience | with arc flash who now never ever stands in front of 480 volt equipment when | he turns it on. He turns it on with a stick while he stands to one side. | Talk about arc flash shy!

I'd agree 480 volts can take the jump across a wider gap that 208 or 240 volts might not. But for a given kVA or kW capacity, the higher voltage system is going to have a higher impedance, reducing fault current capacity. So I would think the arc blast would actually be less due to more of the power being dissipated in smaller wires and windings.

But ... 480 volts also tends to be present when the kVA or kW capacity needs to be higher, or even a lot higher. If you had to decide between 240 and

480 volts for a 300 kW industrial oven, I'm sure you'd go with the 480 volts (if not nore).

I'm not trying to say 480 volts is not as dangerous as you're saying. What I want to say is that 240 volts or even 120 volts can be awfully dangerous itself when high fault currents are available. In a 1500 kVA system, which has the higher current?

My uncle used to do welding with a dozen 2 volt recycled telco lead-acid cells wired in parallel. He had some whopper current there that took days to recharge (he was doing that with solar) and minutes to discharge.

Reply to
phil-news-nospam

| if you are worried about EMF, you are seriously worried about the wrong | thing. There is no serious science that suggests short exposure to such | levels of EMF you experienced has any negative effects. There is a LOT of | experience that says that the work you performed is quite hazardous. the | problem is that while the chance of something going wrong on any individual | exposure to this type of risk is fairly low, the consequences are very | grave, and often fatal. Arc flash is something you should take very | seriously, as it has killed and/or seriously injured a number of people over | the years. You ought to take some time to read up on this, so you will at | least understand how dangerous what you are doing actually is.

EMF can be a damaging force on wiring and systems located ahead of a fault current that doesn't get cleared fast enough, and maybe even when it does.

As for its effect on humans, that I would not worry about since the currents do cancel out and at any distance from the wiring, especially when in a metal conduit, you'd be hard pressed to even detect its presence with instruments.

Reply to
phil-news-nospam

The short circuit current is somewhat independant of voltage, but tends to be higher for higher voltages. Think about it this way. The only thing that limits the current in a phase to phase fault is really the wiring, and

1000 MCM wire has very little resistance.

The load is irrelevant. What creates the dangerous arc fault is the available short circuit current.

Reply to
Bob Peterson

wiring, and

The impedance of the transformer has a big effect on the short circuit. A 2000 kVA transformer 208 V 3 phase transformer with 5% impedance will have a short-circuit current of more than 110,000 A at its secondary terminals (neglecting primary system impedance). 2000 kVA, at 480 V and

5% gives only 48,000 A on a short. For the same power rating, of course the higher voltage uses smaller wire, with a resulting higher branch circuit impedance.

So, at the same transformer power level, I would expect generally that higher voltage distribution systems have a *lower* short-circuit current.

The only place I've ever heard of that couldn't rely on HRC fuses (200 kA interrupting rating) is network power systems with multiple network transformers connected - these have 300+kA fault levels...at 208 V 3 Phase.

Bill

(Bussman publishes a guide to a simplified fault level calculation procedure. )

Reply to
Bill Shymanski

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