industrial panel ground

[CEDEng]

probably better suited in a Industrial Panel Forum somewhere, but I'll try here first...

Small Steel Industrial Panel/Enclosure - incoming power is 120V. Power cord enters thru gland into box.

Ground wire should go where?

At the moment - it is going to a terminal block. The DIN-rail mounted finger safe type. From there, more green wires emanate and land at various power supplies, etc.

(This is not about the number or style of terminal blocks - let's assume that's all in order.)

Should that FIRST green wire - the one right from the wall - be landed at a stud or hard bonded terminal of some type? Or is this "terminal block" distribution method OK?

I cannot find anything (NFPA79, for instance) that gives clear guidance on this, but I am notoriously awful at interpreting their verbiage - so maybe it's clear...somewhere...

Thanks - Input anyone?

 

[iwire]

As far as the NEC there is no 'first place' to land an EGC.

I would be surprised if NFPA 79 is any different.

 

[Smart $]

I agree there is no clear guidance.

I've always assumed the terminal block or bus method as one that will be AHJ approved... and have not had occasion to believe otherwise.

 

[CEDEng]

seems...risky...no? as in, the incidents of terminal blocks not adequately tightened or that work loose or have poor installation, however uncommon, is certainly more than the robust permanently affixed stud or ground bar...but as I said, I cannot find any guidance.

yes, nec has nothing to say about this, because it's inside the box, but NFPA79 is all about inside the box, and still, nothing.

 

[petersonra]

seems...risky...no? as in, the incidents of terminal blocks not adequately tightened or that work loose or have poor installation, however uncommon, is certainly more than the robust permanently affixed stud or ground bar...but as I said, I cannot find any guidance.

yes, nec has nothing to say about this, because it's inside the box, but NFPA79 is all about inside the box, and still, nothing.

terminal blocks are every bit as reliable as stud connections if done correctly. neither is reliable if not done correctly.

studs are not always that reliable either. the stud welder might not be at the right setting or the operator might not have the welding at the correct angle, or pressed hard enough, or did not scrape off the paint first.

all kinds of things can go wrong with either method.

 

[iwire]

seems...risky...no?

No, not at all.

It is just as likely someone does a poor job landing the EGC to the box as the terminal strip

 

[CEDEng]

well, I guess that clears that up - thanks!

i'll add my input just for fun, but I'm considering this resolved:

It's been my experience that a broken stud/poor weld/etc., will be noticed (probably) by the wirer - whereas plenty of folks jam wires into terminal blocks, and do NOT give them the "tug" test, leading to wires that simply fall out. Additionally, the terminal blocks (if more than one required) are "jumped" together with a type of insert-able bridge - which I have seen fall out, and also I have seen plain old forgotten.

none of these things happen with a hard-mounted ground bar, or at least, they tend to be more noticeable.

So - it's not about the reliability with proper care - that part is moot - it's the reliability with little or no care, and that tips me towards the stud.

It's also why I don't write standards for a living : D

 

[just the cowboy]

Strip then stud

Strip then stud

Two things.

Use a grounding terminal strip ( green/yellow ) that grounds each strip to the din rail.
I go to the strip first because in my mind that is more important that you ground your remote devices than the panel stud. If panel stud comes off conduit and mounting usauly ground the box, if you go to stud first and the connection to strip come off any thing mounted to non conductive parts feed with cable is ungrounded.

 

[Smart $]

I've terminated power conductors in all kinds and numbers of industrial panels and enclosures. I would agree that landing on an enclosure welded stud would be preferred, but it seems there is always, for example, a DIN-rail EGC terminal block available right next to where the power circuit conductors land, and then there's another jumpered terminal with a wire EBJ to the enclosure.

FWIW, the wire EBJ is redundant because the grounding terminal block is bonded to the DIN rail which is in turn bonded to the back plate which is secured to the enclosure by four or more studs.

 

[petersonra]

well, I guess that clears that up - thanks!

i'll add my input just for fun, but I'm considering this resolved:

It's been my experience that a broken stud/poor weld/etc., will be noticed (probably) by the wirer - whereas plenty of folks jam wires into terminal blocks, and do NOT give them the "tug" test, leading to wires that simply fall out. Additionally, the terminal blocks (if more than one required) are "jumped" together with a type of insert-able bridge - which I have seen fall out, and also I have seen plain old forgotten.

none of these things happen with a hard-mounted ground bar, or at least, they tend to be more noticeable.

So - it's not about the reliability with proper care - that part is moot - it's the reliability with little or no care, and that tips me towards the stud.

It's also why I don't write standards for a living : D

the "tug" test? a better way to ensure a good connection is to use a proper stripping tool and a torque driver to get the right torque.

as for the stud method, the only truly reliable way of crimping a ring terminal onto the wire is with a controlled cycle tool. I bet a fair number of electricians do not even own one and insist on using a plier type tool, even though they do not make for a reliable connection.

 

[CEDEng]

Thanks everyone, I always learn something here.

The "tug" test, yes - which means, even properly stripped and torqued - if you don't give it a playful tug when done, ya haven't proved a thing.

Many of your comments lead to another question - what if it's not a "grounding" terminal block? In that case, it is not screwed to the dinrail. Is the grounding terminal block required somewhere in the standards?

We build panels here for a living, so our guys do indeed have the proper tools and techniques. We're not (intentionally) unsafe, but it is difficult to Know Everything At All Times, and when people ask me reasonable questions, I have an obligation to research these things!

 

[Smart $]

... Is the grounding terminal block required somewhere in the standards?
...

I'm not versed enough with 79 or UL to say either way with certainty, but I don't believe so.

 

[SceneryDriver]

the "tug" test? a better way to ensure a good connection is to use a proper stripping tool and a torque driver to get the right torque.

I don't care how fancy your wire stripper and and torque screw driver are. You still need to do the tug test. On many types of terminal blocks and devices, it's reasonably possible to get the wire into the terminal, but (sort of, or completely) miss the pressure plate. You can tighten that terminal up to its rated torque, and still have the wire fall out. Ever tried landing wires or troubleshooting in the dark, in a cramped control cabinet? It's easy to do. The tug test never lies.



SceneryDriver

 

[SceneryDriver]

probably better suited in a Industrial Panel Forum somewhere, but I'll try here first...

Small Steel Industrial Panel/Enclosure - incoming power is 120V. Power cord enters thru gland into box.

Ground wire should go where?

At the moment - it is going to a terminal block. The DIN-rail mounted finger safe type. From there, more green wires emanate and land at various power supplies, etc.

(This is not about the number or style of terminal blocks - let's assume that's all in order.)

Should that FIRST green wire - the one right from the wall - be landed at a stud or hard bonded terminal of some type? Or is this "terminal block" distribution method OK?

I cannot find anything (NFPA79, for instance) that gives clear guidance on this, but I am notoriously awful at interpreting their verbiage - so maybe it's clear...somewhere...

Thanks - Input anyone?

We're a 508A panel shop, and we use these:

http://www.automationdirect.com/adc...Terminal_Blocks/Ground_Terminal_Blocks/DN-G10

We stack the needed number on a run of DIN rail, and all the incoming and outgoing ground wires land in the blocks on one side. The other side of the blocks receive the ground wires from all the devices in the panel. We run bonding jumpers from the blocks to other parts of the cabinet, as required; to the door, swing-out panels, etc... Use machine screws in tapped holes (some shops insist on using Tek screws to mount DIN rail) to mount the DIN rail, and remove the paint around the tapped holes in the subpanel to ensure proper electrical continuity.


SceneryDriver

 

[Smart $]

... The tug test never lies.

I wouldn't go as far as saying that... but I agree a tug test provides better assurance than no tug. I say this because I've had occasion to witness a termination in which only one strand made it under the pressure plate. Disintegrated that strand on power up...


...but it passed the tug test. :blink:

 

[SceneryDriver]

I wouldn't go as far as saying that... but I agree a tug test provides better assurance than no tug. I say this because I've had occasion to witness a termination in which only one strand made it under the pressure plate. Disintegrated that strand on power up...


...but it passed the tug test. :blink:

Fair enough. Passed the tug test, but failed the KA-BANG! test. hmy:


SceneryDriver

 

[tom baker]

UL 508 standard requires the torque value for terminals to be labeled inside the panel. For the small din rail terminal blocks, the torque (on what I work on) is usually about 6 in lb.
I go thru with a Whia 3 mm screwdriver and then follow up with a torque screwdriver, I almost always get another 1/2 turn on the torque screwdriver. The best torque screwdriver I have found is Whia, http://www.wihatools.com/torq-tools-catalog
All the control panels from various integrators I install have din rail mounted terminals and ground terminals, none have a connection to a ground stud. But the back panel has star washer under one of the mounting studs

 

[iwire]

the torque (on what I work on) is usually about 6 in lb.
I go thru with a Whia 3 mm screwdriver and then follow up with a torque screwdriver, I almost always get another 1/2 turn on the torque screwdriver.

We are all so different, every time I use a torque tool I find I have been over tightening pretty much everything.

 

[petersonra]

We are all so different, every time I use a torque tool I find I have been over tightening pretty much everything.

most people do. tighter is better, even though it is not.