Grounding probes: Hazard or helpful?

[BeanAnimal]

<a href=showthread.php?s=&postid=13418344#post13418344 target=_blank>Originally posted</a> by stugray
Actually, in YF's defense....

While thinking through what he was saying ( as applied to transient surge protection NOT breakers or appliances ),

I thought of one case where it would be BAD for the surge supressor to be able to conduct MORE surge than the MAIN BREAKER is capable of interrupting ( This is the AIC rating).


IF you installed a HUGE surge supressor on the House side of the main breaker, AND it absorbed 70kA of current ( microseconds to milliseconds depnding on the "surge"), when the Main breaker is only rated at say...30kA, then it is possible to weld the Main Breaker's Contacts closed.

This would neutralize the Main breakers Interrupt Capacity for future faults, and the homeowner would never know it.

This is the ONLY case where I can see not wanting the downstream side components to NOT have greater AIC than the service feed.


However, I applaud YF's remark above. Thank you.

Stu

Stu, there is only one problem with that scenario... (you knew I had an answer, didn't you).


The same transient WITHOUT the TVSS in place would do the same damge to the main breaker AND likely damage some, many, or all of the devices connected to the service because the transient was NOT clamped. The homeowner would only know that he lost electronic gear in the home, not the main breaker. That is exactly why the NFPA/NEC views TVSS devices they way they do. The paper I linked to covers this very logic

As far as Frank admitting he was wrong... I almost fell outa my seat ... Though I am still puzzled why the NEC, Florida Power and Light, 6 Engineers from the Cooper Bussman Company, a Cuttler Hammer engineer, you and me were/are not enough proof and he still wants to contact a few more people to make sure

I guess the fact that he said he may be wrong, pending a few more phone calls is better than where we were a page ago. Maybe Steve has turned over a new leaf and we can avoid all this in the future and instead all learn from each other.

 

[BeanAnimal]

Stu (you knew I had more on the subject)...

It may also be useful at this juncture to point out that the AIC rating of a device is usually much lower than the SCCR rating of the device. We have been talking about the AIC rating to this point. The SCCR rating follows the same logic with regard to bigger is better and allowed. However, your scenario does beg a broader explanation of what we are looking at.

SCCR is the Short-Circuit Current Rating. It refers to the maximum amount of short circuit current that a device can sustain and survive.

AIR is the Ampere Inturrupting Rating. It refers to the maximum current that a device can survive WHILE trying to open the circuit.

Why is the SCCR usually so much higher than the AIC rating? Simply put, Opening a circuit against a short is often a much more abusive task than simply carrying the current for a short period of time. I.E. Sustaining a load on closed contacts is not damaging. Opening those contacts under load wil cause an arc. At high current, the arc is MUCH more apt to cause the device to explode into shrapnel!

 

[Haxer]

For what it's worth BeanAnimal, i work at Eaton Electrical (Cutler Hammer) and i agree with you on the previous posts, we make SwitchGear, Network Protectors, Busway, you name it... Utilites put our Network Protectors in underground concrete vaults, because like a TVSS ... if you get to much current ... it aint pretty ...

 

[stugray]

BeanAnimal,

"The same transient WITHOUT the TVSS in place would do the same damge to the main breaker AND likely damage some, many, or all of the devices connected to the service because the transient was NOT clamped"

Agreed.

SO.. would it be prudent to put a very high current rating TVSS with a Gas Tube supressor ( IN SERIES ) BEFORE the breaker?

That way it would dissipate the surge before the Main Breaker ( saving it wear-and-tear ), but in a Failed-Short situation, the Gas Tube would blow ( Fuse )?

Stu

 

[Young Frankenstein]

<a href=showthread.php?s=&postid=13419323#post13419323 target=_blank>Originally posted</a> by Haxer
For what it's worth BeanAnimal, i work at Eaton Electrical (Cutler Hammer) and i agree with you on the previous posts, we make SwitchGear, Network Protectors, Busway, you name it... Utilites put our Network Protectors in underground concrete vaults, because like a TVSS ... if you get to much current ... it aint pretty ...

say hello to Nacho

 

[BeanAnimal]

Thanks Haxer

Stu, here is the way I see it..

If the surge is large enough to damage the main breaker in even a modest service, it is likely not going to be clamped by any reasonably sized or priced TVSS. Remember the SCCR is going to be larger than the worst case fault current from the transformer. So to exceed the SCCR rating of the equipment we need an excess of voltage and/or current that is otherwise unavailable even in a direct short condition. So what can cause a voltage and current to exceed what is calculated as WORST CASE. Well only two things that I know of... well three....

1) Lightening - a non direct strike someplace in the system will likely be dissapated through many other service drops on the same system. This is where the TVSS can help and no further damage is likely to be encountered. We also know that a direct strike on the service or service equipment is likely going to jump a gas tube, TVSS or whatever it damn well pleases. In that case all bets are off, so who cares what the fault current ratings are, they are going to be exceeded by magnitudes.

2) A car crash, tree or other situation that causes an HV line to contact a lower voltage feed or tap, or an HV transformer failure. The hope here is that the safeguards on the grid will handle most of this fault current before it reaches our meager little TVSS and service panel. If it does hit us, all bets are off and molten metal is going to be the result no matter what we have in place.

3) A very large EMP caused by a thermonuclear device detonation... all bets are off...

No back to the reasoning behind the code. In a nutshell, the TVSS is not going to clamp anything that is going to blindly damage the main breaker. Currents that are large enough to do that kind of damage are going to blow stuff up with our without the TVSS (before or after the main breaker).

Just my .02

Bean

 

[Young Frankenstein]

Ge engineer " Same or above"
All South Lightning protections engineer
" Same or above"
One more question was asked about lightning protection and TVSS on length of wire.
This is what he said.

The reason most systems have them built in panels is wire length.
12" leads and above will "let throe" enough surge to do major damage.

So I was wrong on the Fault current design.

Theres much more interesting things about lightning protection. We talked in the shop for about 1 hr.

 

[BeanAnimal]

<a href=showthread.php?s=&postid=13432374#post13432374 target=_blank>Originally posted</a> by Young Frankenstein
One more question was asked about lightning protection and TVSS on length of wire.
This is what he said.

The reason most systems have them built in panels is wire length.
12" leads and above will "let throe" enough surge to do major damage.

I think I mentioned that a few times above when answering Stu's questions... Maybe I meant to, but never typed it. Yes, the shorter the path the the fault clamp, the more likely it is to take the bulk of the current before that current finds OTHER paths to ground.

Stu had asked about putting it on the service entrance. That topology does have some merit, but the odd reality is that most damage is from small spikes that originate in house, not at the other side of the meter! So we (in most cases) put the TVSS as close to the panel bus as possible to prevent in house spikes from find a path from the originating branch to other branches on the bus. We also have fair protection against lightening, etc. On the other hand of the TVSS was at the meter socket, we would lose the "in house" protection from branch circuit to branch circuit.

So I was wrong on the Fault current design.

Thank you Steve. See how easy that was... I hate being wrong too... but that is also how I learn things.

Theres much more interesting things about lightning protection. We talked in the shop for about 1 hr.

Yes, it is a very interesting subject. As I mentioned, lightening and pole generated events are only part of what we worry about. In house surges do a lot of damage as well. As you harmonic problems can be hard to track down and hard to fix.

Thanks again for stepping up Steve.