# Grounding resistors and dust



## MDShunk (Jan 7, 2007)

How hot will grounding resistors get during a fault, and for how long? I was at a place yesterday that had a few banks of them mounted on the wall. They were the filthiest grounding resistors I've ever seen. Literally an inch of dust on them. My mind started to wonder if a fault was occurring below the trip threshold (or, below the charging current of the system), could these resistors catch the dust on fire and burn out the resistors completely?


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## drsparky (Nov 13, 2008)

They would not dissipate heat properly, probably burn up. I think flames depend on what the dust is made of.


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## oldtimer (Jun 10, 2010)

MDShunk said:


> How hot will grounding resistors get during a fault, and for how long? I was at a place yesterday that had a few banks of them mounted on the wall. They were the filthiest grounding resistors I've ever seen. Literally an inch of dust on them. My mind started to wonder if a fault was occurring below the trip threshold (or, below the charging current of the system), could these resistors catch the dust on fire and burn out the resistors completely?



IMO, if left like that,they will eventually burn up. They cannot dissipate the heat properly. Also, depending on what the dust is, would they not eventually short out? :blink:


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## micromind (Aug 11, 2007)

I'd say about 1/2 of the grounding resistors I've seen will handle the voltage/current across them at the relay trip threshold indefinitely. Covered with dust though, very likely it couldn't dissipate enough heat anymore.

Based on how it's constructed, if a resistor burns up, it will almost certainly be open-circuit. Which would cause full voltage to be applied to the relay, resulting in a trip.

I haven't seen a grounding resistor burn up yet, so I don't know if it would ignite dust or not. I suspect it would though. I have seen small ceramic resistors (10 watt or so) be overloaded and glow red. Seems to me that would be hot enough to ignite dust.

Rob


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## RIVETER (Sep 26, 2009)

MDShunk said:


> How hot will grounding resistors get during a fault, and for how long? I was at a place yesterday that had a few banks of them mounted on the wall. They were the filthiest grounding resistors I've ever seen. Literally an inch of dust on them. My mind started to wonder if a fault was occurring below the trip threshold (or, below the charging current of the system), could these resistors catch the dust on fire and burn out the resistors completely?


I guess I haven't seen a system like that. Are you talking about a hi-resistance grounding of an electrical system and you are monitoring for ground fault smaller currents?


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## oldtimer (Jun 10, 2010)

MDShunk said:


> How hot will grounding resistors get during a fault, and for how long? I was at a place yesterday that had a few banks of them mounted on the wall. They were the filthiest grounding resistors I've ever seen. Literally an inch of dust on them. My mind started to wonder if a fault was occurring below the trip threshold (or, below the charging current of the system), could these resistors catch the dust on fire and burn out the resistors completely?





Do you mean neutral grounding resistors? Used to protect transformers etc.


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## MDShunk (Jan 7, 2007)

RIVETER said:


> I guess I haven't seen a system like that. Are you talking about a hi-resistance grounding of an electrical system and you are monitoring for ground fault smaller currents?





oldtimer said:


> Do you mean neutral grounding resistors? Used to protect transformers etc.


They were the regular GE DS9181 panels that you see on all kinds of gear installed for the last 20-30 years. There were several of them on this particular lineup, so I'm not sure if it was for the gear itself or for a few large motor loads. Yes, it was high resistance grounding. These were wall mounted, with the resistor racks wall mounted above them. What was remarkable about this installation was that the gear rooms were so clean, but these resistors had an incredible amount of dust and lint piled up on top of them. I was there for another reason, so it really wasn't my deal. The resistors are pretty easy to see, since they're more or less mounted inside a "cage".

I didn't really inspect things very carefully. This particular room had 6 rows of gear, and almost 30 rows of MCC buckets.


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## RIVETER (Sep 26, 2009)

MDShunk said:


> They were the regular GE DS9181 panels that you see on all kinds of gear installed for the last 20-30 years. There were several of them on this particular lineup, so I'm not sure if it was for the gear itself or for a few large motor loads. Yes, it was high resistance grounding. These were wall mounted, with the resistor racks wall mounted above them. What was remarkable about this installation was that the gear rooms were so clean, but these resistors had an incredible amount of dust and lint piled up on top of them. I was there for another reason, so it really wasn't my deal. The resistors are pretty easy to see, since they're more or less mounted inside a "cage".
> 
> I didn't really inspect things very carefully. This particular room had 6 rows of gear, and almost 30 rows of MCC buckets.


I had heard many years ago of systems that would operate similar to un-grounded systems but could make it easier to find ground faults. However, being resistance grounded they posed a 24/7 I2R loss. So some went to impedance grounded instead. But I've not worked on one.


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## JohnR (Apr 12, 2010)

What voltage system were they on? From my training, yes that can cause a fire, but the question is when. Could be scheduled for the next PM. 

There is a guy out there that even cleans HIGH voltage that has dust, tracking, and such like. OH........ LIVE 

Something about compressed air and walnut shells. No one knows how he does it because he won't let anyone watch him.

I guess he don't want to think that if his trainee got hurt it would be his fault.


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## Shorty Circuit (Jun 26, 2010)

MDShunk said:


> How hot will grounding resistors get during a fault, and for how long? I was at a place yesterday that had a few banks of them mounted on the wall. They were the filthiest grounding resistors I've ever seen. Literally an inch of dust on them. My mind started to wonder if a fault was occurring below the trip threshold (or, below the charging current of the system), could these resistors catch the dust on fire and burn out the resistors completely?


That's not dust. It's all that's left of the last guy who tried to clean them hot. :laughing:


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## crue (Mar 29, 2013)

it depends on protection degree.
check this out.
.................http://www.hilkar.com/neutralgroundingresistors.html


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## Southeast Power (Jan 18, 2009)

Not to worry, a big load will shake the dust off of them.

I installed some outside of a building near a tree, they are now picnic tables.

Btw,
The nuisance tripping stopped on the 4160 generator they are connected to.


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