dslreports logo
site
 
    All Forums Hot Topics Gallery
spc

spacer




how-to block ads


Search Topic:
uniqs
1783
share rss forum feed


Jack_in_VA
Premium
join:2007-11-26
North, VA
kudos:1

1 recommendation

What happens when an overcurrent device doesn't open



jack b
Gone Fishing
Premium,MVM
join:2000-09-08
Cape Cod
kudos:1
Maybe the current draw was less than the over-current device rating.
If the fault was lets say 10 ohms, then the current draw would be 1440 watts at 120 volts, or 12 amps. Not nearly enough to trip a 15 amp breaker.
--
~Help Find a Cure for Cancer~
~Proud Member of Team Discovery ~

Speedy Petey

join:2008-01-19
said by jack b:

Maybe the current draw was less than the over-current device rating.
If the fault was lets say 10 ohms, then the current draw would be 1440 watts at 120 volts, or 12 amps. Not nearly enough to trip a 15 amp breaker.

My thought exactly. It wouldn't take much ground fault amperage over a long period of time to do that.


Jack_in_VA
Premium
join:2007-11-26
North, VA
kudos:1
I agree any current under 15 amp would over time allow a lot of heat.


jack b
Gone Fishing
Premium,MVM
join:2000-09-08
Cape Cod
kudos:1

2 edits
An arc fault breaker/interrupter also might not have activated under these conditions.

Zach1
Premium
join:2006-11-26
NW Minnesota
reply to Jack_in_VA
A perfect illustration as to why I always include a properly terminated equipment grounding conductor of the correct size. IMHO, EMT is inadequate as an equipment grounding path regardless of what the NEC states.
--
Zach


Jack_in_VA
Premium
join:2007-11-26
North, VA
kudos:1
Reviews:
·Millenicom
said by Zach1:

A perfect illustration as to why I always include a properly terminated equipment grounding conductor of the correct size. IMHO, EMT is inadequate as an effective equipment grounding path regardless of what the NEC states.

We required a grounding conductor in all conduits. We had Aluminum, galvanized, stainless steel, PVC covered galvanized and plain PVC. In short so many variables it was just easier to require the grounding conductor.

Zach1
Premium
join:2006-11-26
NW Minnesota
said by Jack_in_VA:

said by Zach1:

A perfect illustration as to why I always include a properly terminated equipment grounding conductor of the correct size. IMHO, EMT is inadequate as an effective equipment grounding path regardless of what the NEC states.

We required a grounding conductor in all conduits. We had Aluminum, galvanized, stainless steel, PVC covered galvanized and plain PVC. In short so many variables it was just easier to require the grounding conductor.

Absolutely couldn't agree more strongly. Grounding conductors all the time every time with the only exceptions that come to mind being a service risers and service entrance conduits. Locally, IMC or RMC is required for service entrance conduits so, bond bushings are used regardless of KO type. I've seen way too many loose set-screws, lock-nuts, improper repairs, damaged raceway and poor installation practices to ever consider depending on a raceway for anything more than a supplementary grounding path.
--
Zach

garys_2k
Premium
join:2004-05-07
Farmington, MI
Reviews:
·Callcentric
I did a fair amount of factory engineering work some years ago and the electricians would not install a dedicated ground conductor in a hard-piped conduit run. This wasn't EMT, but the real rigid conduit. There never was any problems, but none of this was in explosive atmosphere areas.


Lurch77
Premium
join:2001-11-22
Oconto, WI
kudos:4

1 recommendation

reply to Jack_in_VA
quote:
Uh oh...
It looks like our servers are currently over capacity. Check back in a minute or two and the site should be back to normal.
Apparently too many people hot linking to them.

iknow
Premium
join:2012-03-25
reply to garys_2k
said by garys_2k:

I did a fair amount of factory engineering work some years ago and the electricians would not install a dedicated ground conductor in a hard-piped conduit run. This wasn't EMT, but the real rigid conduit. There never was any problems, but none of this was in explosive atmosphere areas.

a little rust or corrosion on a joint, and you'd have what you see in the first picture!.


nunya
LXI 483
Premium,MVM
join:2000-12-23
O Fallon, MO
kudos:12
Reviews:
·Charter
·voip.ms
·surpasshosting
reply to garys_2k
I normally run a EGC in EMT, but I rarely run one in IMC or heavywall. If it's spec'd, I'll pull it. If not, I don't waste the wire. The conduit is a fine ground.
A good engineer will calculate the conduit size, length, and type compared to the over-current protection. This can save customers thousands of dollars in wire and labor, with zero additional risk.

All the connections are threaded tight. If you are doing the minimum 5 on pipe tapered to code requirements, a little rust or corrosion isn't going to be a problem. I've broken up 70 year old weathered joints that are still fresh as a daisy while the outside looks like total ass. Why? because it was done right.

Hell, even on small jobs I can look in the Soares book and get my answer in a few minutes. It's not rocket science.

The only reason I pull one in EMT is that it tends to be abused over the years. Some places use it as a long storage hanger. "Handymen" mess with it and leave it in a fubar state.
I can't tell you how many times I've happened into a piece of EMT that's separated at a joint. The "compression" type couplings and connectors actually fall apart way more than the set screw.
--
If someone refers to herself / himself as a "guru", they probably aren't.


Jack_in_VA
Premium
join:2007-11-26
North, VA
kudos:1
Reviews:
·Millenicom
said by nunya:

I

All the connections are threaded tight. If you are doing the minimum 5 on pipe tapered to code requirements, a little rust or corrosion isn't going to be a problem. I've broken up 70 year old weathered joints that are still fresh as a daisy while the outside looks like total ass. Why? because it was done right.

Are you considering cost to be more important than overall safety over years of use and ignoring environmental aspects of where the conduit is installed? Corrosion and rust are huge factors that must be considered. The conduit can't be a ground when it's more or less eat in two.

We required the grounding conductor in every conduit. No exceptions. We put safety ahead of cost which was really not that much.

Conduit threads are straight as opposed to pipe threads which are tapered and the tightness depends on the ends being cut straight so they can butt in couplings and fittings. I've seen short mis-measured conduit runs where the difference is made up by unscrewing it to the extent of only a couple of threads are holding it together.


Cho Baka
Premium,MVM
join:2000-11-23
there
kudos:2
reply to Jack_in_VA
Nunya described a balance of cost/overkill based on experience/judgment and code.

Balance is the name of the game.
--
The talented hawk speaks French.


Jack_in_VA
Premium
join:2007-11-26
North, VA
kudos:1
Reviews:
·Millenicom
said by Cho Baka:

Nunya described a balance of cost/overkill based on experience/judgment and code.

Balance is the name of the game.

Safety is the name of the game. You're welcome to your opinion but there is no cost too great for safety especially if you know you can make something safer.


Cho Baka
Premium,MVM
join:2000-11-23
there
kudos:2

1 recommendation

If there is no cost too great, then why not spec out stainless conduit and double the wire gauge just in case?

Common sense and cost are why.
--
The talented hawk speaks French.


Jack_in_VA
Premium
join:2007-11-26
North, VA
kudos:1
Reviews:
·Millenicom
said by Cho Baka:

If there is no cost too great, then why not spec out stainless conduit and double the wire gauge just in case?

Common sense and cost are why.

We did spec out PVC coated conduit, Stainless Steel conduit, fittings and hangers.

Like I said there is no cost limits for safety if you know there is a better way.


nunya
LXI 483
Premium,MVM
join:2000-12-23
O Fallon, MO
kudos:12
Reviews:
·Charter
·voip.ms
·surpasshosting

1 recommendation

reply to Jack_in_VA
Your guys were installing conduit improperly, as code requires threads to be tapered 3/4" / ft (not optional). Your guys were using the wrong dies. Also, conductive thread sealing compound is a must (I use Appleton Electric TLC3, it's about $20 a tube). On tower sites they demand some really nasty zinc / copper stuff, but the Appleton stuff meets code requirements. Furthermore, if you have conduit being "eaten in two", then you obviously have other issues that need to be addressed.
Everything you've cited so far is the result of an improperly installed system.
Your company can require whatever they want, they are paying for it. I'd take the extra money you guys waste on wire and train your electricians how to install a proper conduit system.
--
If someone refers to herself / himself as a "guru", they probably aren't.

garys_2k
Premium
join:2004-05-07
Farmington, MI
Reviews:
·Callcentric
Yes, the pros where I worked, the factory where only rigid conduit was used without ground wires, always used threaded ends (the tripod mounted power cutter took maybe 10 seconds to thread them) and the conductive goop. This was almost all 480 volt, or higher, for inductive loads (motors) and feeds to transformers feeding 120 volt panels.


PSWired

join:2006-03-26
Annapolis, MD
reply to Jack_in_VA
Of course there are cost limits for safety. Every industry makes judgement calls about the cost/benefit for a given piece of safety equipment. The decision to install a traffic signal in lieu of stop signs, for example, is essentially governed by a threshold for deaths prevented by the signal. If cost were no object, we'd have railings in the subway, guardrails on all our roads, and GFCIs at every outlet.


Jack_in_VA
Premium
join:2007-11-26
North, VA
kudos:1
Reviews:
·Millenicom
said by PSWired:

and GFCIs at every outlet.

You have to know it's close to that now when you add AFCI's.


whizkid3
Premium,MVM
join:2002-02-21
Queens, NY
kudos:9

1 edit
reply to nunya
said by nunya:

I normally run a EGC in EMT, but I rarely run one in IMC or heavywall. If it's spec'd, I'll pull it.The conduit is a fine ground. A good engineer will calculate the conduit size, length, and type compared to the over-current protection. This can save customers thousands of dollars in wire and labor, with zero additional risk.

Generally I spec ground wires in every type of raceway. Often, the building owner makes a decision not to have the ground wires to save costs. Depends on the type of project and system. Some systems (IT, modern elevators, UPS, VFDs, etc) call out for having a separate ground wire to lower the impedance to ground, over that of steel conduit. I hadn't thought about it, but absolutely requiring a separate ground in EMT is a good idea.

As to calculating whether or not the steel conduit provides an adequate fault path to ground (the impedance of hte conduit), its a rare engineer that actually does it. Tables are available from conduit manufacturers, as to what is the maximum distance of different size conduits, before the ground ability of the conduit becomes unsuitable. These should be used on any job where the steel conduit is being used as the ground path.

said by nunya:

The only reason I pull one in EMT is that it tends to be abused over the years. Some places use it as a long storage hanger. "Handymen" mess with it and leave it in a fubar state.
I can't tell you how many times I've happened into a piece of EMT that's separated at a joint. The "compression" type couplings and connectors actually fall apart way more than the set screw.

Agreed as to how many times I've seen EMT separated at set-screw connectors. I haven't seen compression couplings fall apart; and in my opinion, this only happens if the coupling wasn't wrench-tightened. Its best to literally spec 'wrench-tightening', and prohibit set-screw couplings, IMHO. I believe EMT compression couplings offer quite a few advantages over set-screw couplings, including use in wet environments, and providing better protection against radiated noise via the concentric shielding around the conductors from end to end. Of course, it pays to check if the contractors left the fittings loose, or actually tightened them. I have seen plenty of times where they did not, and were either forgetful or hoped someone wouldn't notice.

said by jack b:

Maybe the current draw was less than the over-current device rating.

Nice photo, jack. I've never seen anything like that. Its a good reason, IMHO, not to use set-screw fittings on EMT. The coupling is literally melted.


PSWired

join:2006-03-26
Annapolis, MD
reply to Jack_in_VA
True indeed.