Terminating to a spare breaker while energized?

[K8MHZ]

More of us get killed by 120 volts than any other voltage.

By 'us' do you mean humans or electricians?

 

[iwire]

By 'us' do you mean humans or electricians?

In the US I mean both and yes I realize that it is the most common voltage we run into.

The point was 120 can and does kill.

 

[PetrosA]

I'm trying to figure out how to join in the conversation without adding any fuel to anyone's fire I realized that every time this topic comes up, I find myself cheering both sides of the argument, as strange as that sounds, and it's encouraged me to do some more digging for information online. One of the things about these arguments I've noticed is that the regulations are there to protect against both arc-flash and shock hazards but the lines get blurred here in the forum. I don't think there are many electricians who work in residential or light commercial (300V or less, low amperage systems) who have seen or dealt with an arc-flash incident. Sparks, yes, arc-flashes not so much. Guys who deal with larger systems feel the danger a whole lot more. Because of that, I think there's a lot of resistance to talk about arc-flash danger and the imposed mitigation techniques from the guys who aren't feeling threatened and who work in environments where mitigation is not part of a daily routine. Most residential or light commercial customers will be very unpleasantly surprised and confused by the news that all power must be shut down for what seem like simple tasks while in other cases POCOs may not support a de-energizing policy or the fact that multiple customers would be affected would make de-energizing much more complicated. On the other hand, resistance to PPE and safe work practices from the resi/lightcom guys is stupid when viewed from a shock hazard perspective.

I do think that applying really strict (no energized work - period) rules to less than 300V nominal residential systems is counter-productive lawmaking for a number of reasons. If the shock hazard can be mitigated with proper PPE and training provided, I feel that live work can be done safely and should be allowed on these systems since the arc-flash hazard is minimal (based on the statistics for arc-flash incidents, or lack of them, for these systems found online). Regularly pulling meters is not a safe option since many of the older meter sockets may pose a much greater arc hazard when the meter is pulled than the worker would be exposed to in a fully energized panel. As many of us have noticed, not all POCOs are timely about hooking up customers when called and some (like the one around here) won't even send a crew to reconnect an aerial service - that's the electrician's job. I can't call in a broken neutral unless I've confirmed that the connections in the meter base are good which you can't do de-energized, nor can the "boss" run around and be the one doing what OSHA forbids employees to do. Finally, complete bans on energized work create a dangerous environment of "under the radar" work habits that are even more likely to ignore PPE for shock hazard, ultimately causing greater risks to workers.

I've never seen, heard of or read of an arc-flash incident on a residential or light commercial system, but I have been shocked, I've known a lot of other guys who have been shocked and I remember every single time a local electrician was killed by electrocution. In spite of that, there still isn't one supply house or box store in a fifty mile radius of where I live and work that sells any kind of PPE for shock hazard mitigation. The ONLY place I know locally to buy any kind of PPE is Grainger. Some 1000V tools are easily available but their quality is questionable. Rubber mats, face shields, FR clothing? All special order. For small contractors, conforming isn't so black and white and what's worse, lack of available PPE makes mitigating shock hazard difficult. That's why I cheer the stronger regulations. They'll eventually make it easier to work safely. In the meantime, it just feels like I'm peeking through the looking glass when I read these threads because what I see just totally looks like a separate reality.

 

[zog]

Well said Peter, very well said. I tend to be on the safe side because I am a high power guy, power plants, very heavy industrial, etc.. Never done a resi job in my life and don't worry about arc flash in my home either. The hard question is, where exactly do you draw the line?

 

[the blur]

The hard question is, where exactly do you draw the line?

and the saftey guys will tell you to perform an AF study on every panel you open up.

Hey Mrs. Jones, I need to do some calculations before I can open up your electrical panel.
I'm sorry Mrs. Jones, we need to call the power company to shut down your entire house.


But yes, that was well said, and I think any line will be drawn in the "SAND". This way the gov't can move the line either direction on a whim.

 

[zog]

and the saftey guys will tell you to perform an AF study on every panel you open up.

.

I sure hope not. If so they are clueless.

 

[the blur]

Actually you can forget about the AF study, and just call the power company to pull the meter.

Maybe one day, someone will write a new NFPA for residential and light commerical 208Y. But again, they will tell you to perform a study, because every pad mount has different potential.

I will tell you, since all this, I stopped doing 480 work for the most part. I lost the comfort level I had. Even though most of the accidents are from cheap meters, or dropping tools.

 

[the blur]

I just watched the TV show, "HOW IT'S MADE".

New electric meters. the power company can remotely shut off the house with a wireless command.

But I wonder what the intended use is ? and if they will roll it out to all existing customers, or only new construction.

 

[texie]

I'm trying to figure out how to join in the conversation without adding any fuel to anyone's fire I realized that every time this topic comes up, I find myself cheering both sides of the argument, as strange as that sounds, and it's encouraged me to do some more digging for information online. One of the things about these arguments I've noticed is that the regulations are there to protect against both arc-flash and shock hazards but the lines get blurred here in the forum. I don't think there are many electricians who work in residential or light commercial (300V or less, low amperage systems) who have seen or dealt with an arc-flash incident. Sparks, yes, arc-flashes not so much. Guys who deal with larger systems feel the danger a whole lot more. Because of that, I think there's a lot of resistance to talk about arc-flash danger and the imposed mitigation techniques from the guys who aren't feeling threatened and who work in environments where mitigation is not part of a daily routine. Most residential or light commercial customers will be very unpleasantly surprised and confused by the news that all power must be shut down for what seem like simple tasks while in other cases POCOs may not support a de-energizing policy or the fact that multiple customers would be affected would make de-energizing much more complicated. On the other hand, resistance to PPE and safe work practices from the resi/lightcom guys is stupid when viewed from a shock hazard perspective.

I do think that applying really strict (no energized work - period) rules to less than 300V nominal residential systems is counter-productive lawmaking for a number of reasons. If the shock hazard can be mitigated with proper PPE and training provided, I feel that live work can be done safely and should be allowed on these systems since the arc-flash hazard is minimal (based on the statistics for arc-flash incidents, or lack of them, for these systems found online). Regularly pulling meters is not a safe option since many of the older meter sockets may pose a much greater arc hazard when the meter is pulled than the worker would be exposed to in a fully energized panel. As many of us have noticed, not all POCOs are timely about hooking up customers when called and some (like the one around here) won't even send a crew to reconnect an aerial service - that's the electrician's job. I can't call in a broken neutral unless I've confirmed that the connections in the meter base are good which you can't do de-energized, nor can the "boss" run around and be the one doing what OSHA forbids employees to do. Finally, complete bans on energized work create a dangerous environment of "under the radar" work habits that are even more likely to ignore PPE for shock hazard, ultimately causing greater risks to workers.

I've never seen, heard of or read of an arc-flash incident on a residential or light commercial system, but I have been shocked, I've known a lot of other guys who have been shocked and I remember every single time a local electrician was killed by electrocution. In spite of that, there still isn't one supply house or box store in a fifty mile radius of where I live and work that sells any kind of PPE for shock hazard mitigation. The ONLY place I know locally to buy any kind of PPE is Grainger. Some 1000V tools are easily available but their quality is questionable. Rubber mats, face shields, FR clothing? All special order. For small contractors, conforming isn't so black and white and what's worse, lack of available PPE makes mitigating shock hazard difficult. That's why I cheer the stronger regulations. They'll eventually make it easier to work safely. In the meantime, it just feels like I'm peeking through the looking glass when I read these threads because what I see just totally looks like a separate reality.

You make some very good points. I see both sides of this fence. As one who has been in the trade a long time, when I think back to some of the "hot" work we used to do on a regular basis I shudder. Accidents such as this:
http://www.naplesnews.com/news/2011/dec/15/explosion-injures-fpl-worker-naples-bay-resort-off/ happen way to often and serve to make you think.

 

[pfalcon]

and the saftey guys will tell you to perform an AF study on every panel you open up. ...

The value at the last limiting device can be used for the arc flash. Line resistance from that point to the panel will reduce the risk. For example it might lower the fault current from 10kA to 9kA but you can stick with the 10kA value for the arc flash calculations, effectively assuming no resistance from the limiting device to the panel. This would allow the POCO to post the AF at the meter base without individual analysis.

I just watched the TV show, "HOW IT'S MADE".
New electric meters. the power company can remotely shut off the house with a wireless command.
But I wonder what the intended use is ? and if they will roll it out to all existing customers, or only new construction.

Wonder no longer. The concept was created to avoid rolling brownouts. Big Brother will shut off whole neighborhoods or every third house through a rationing process. So for example - during a 90 degree day they'll shut your power off for one hour at 3am 6am 9am 12noon 3pm 6pm 9pm 12midnight. Thereby guaranteeing that from 4pm to 6pm your AC will run constantly and never catch up to cool the house. This will guarantee maximum electric bills with maximum wear and tear on the equipment and minimal effectiveness of the AC unit. And you'll undoubtedly pay a user rate fee for the privilege.

After consideration was given to the possible fire hazards and equipment damage possible (and grandma's breather being unpowered) by cycling the whole home power supply they came up with the next Great Idea - Smart furnaces and Smart AC. A special box would be installed that would disable the furnace or AC on command instead of the whole house. A box probably with the same quality as your cable TV box and installed with anti-tampering to guarantee at least 2 days before a service tech arrives. But it's still up in the air which way to go because it's much easier to have the POCO alter the meters than it is to rewire everyone's furnace circuits.

Sadly, none of the above is sarcasm. :happysad:

 

[the blur]

So they fit a 200amp contactor in the meter.....

 

[jim dungar]

The value at the last limiting device can be used for the arc flash. Line resistance from that point to the panel will reduce the risk.

This is not true.
If the line resistance lowers the fault current below the current limiting point of the protective device, the incident energy may increase substantially, due to the longer clearing time.

 

[texie]

This is not true.
If the line resistance lowers the fault current below the current limiting point of the protective device, the incident energy may increase substantially, due to the longer clearing time.

Agreed, though to many I think this seems counter intuitive. I think part of the problem is the lack of understanding in general of available fault current and incident energy. It seems to me that we in the industry don't spend nearly enough time on the how and why of arc flash. I believe the vast majority of electricians work in live panels with very little understanding of the risk. It's part of the culture.

 

[petersonra]

Agreed, though to many I think this seems counter intuitive. I think part of the problem is the lack of understanding in general of available fault current and incident energy. It seems to me that we in the industry don't spend nearly enough time on the how and why of arc flash. I believe the vast majority of electricians work in live panels with very little understanding of the risk. It's part of the culture.

I think a lot of them work live because the arc flash risk is very low in most commercial and residential situations.

The problem is that every now and then the risk is not nil in such a situation.

There is just not a good answer to this problem at present.

 

[pfalcon]

This is not true.
If the line resistance lowers the fault current below the current limiting point of the protective device, the incident energy may increase substantially, due to the longer clearing time.

Okay, before replying I went back through the device data sheets. For example, our typical fuse is Bussmann LPS-RK30SP. To drop below the current limiting point of the device requires dropping below 500A.

Bussmann lists the incident energy to be .25cal/cm2 with a 6" boundary.

Other system specs are 480Vac with worst case bolted fault current from the preceeding transformer of 75kA.

Per your statement I should be worried about the line length from the transformer to this buss head changing the incident energy and PPE requirements? Or are you just postulating a case where someone failed to size conductors for the distance?

 

[jim dungar]

Okay, before replying I went back through the device data sheets. For example, our typical fuse is Bussmann LPS-RK30SP. To drop below the current limiting point of the device requires dropping below 500A.

Bussmann lists the incident energy to be .25cal/cm2 with a 6" boundary.

Other system specs are 480Vac with worst case bolted fault current from the preceeding transformer of 75kA.

Per your statement I should be worried about the line length from the transformer to this buss head changing the incident energy and PPE requirements?

I was pointing out that you cannot make a blanket statement that downstream equipment will always have lower PPE requirements than at an upstream current limiting fuse.

Actually, it takes about 305A to have a 35A LPS-RK1 fuse enter its 'incident energy reduction' region for <1.2 cal/cm?, so this small size may not be the best fuse to use in this discussion.
I just finished an exercise to demonstrate how critical it is to not estimate conductor length. With a 110A RK1 fuse an additional 30' of conductor meant going from <1.2cal/cm? up to >8.0 cal/cm?.

 

[pfalcon]

I was pointing out that you cannot make a blanket statement that downstream equipment will always have lower PPE requirements than at an upstream current limiting fuse.

Actually, it takes about 305A to have a 35A LPS-RK1 fuse enter its 'incident energy reduction' region for <1.2 cal/cm?, so this small size may not be the best fuse to use in this discussion.
I just finished an exercise to demonstrate how critical it is to not estimate conductor length. With a 110A RK1 fuse an additional 30' of conductor meant going from <1.2cal/cm? up to >8.0 cal/cm?.

305A, 500A, Whatever it takes I was just eyeballing a data sheet eye-test graph from Bussmann for the 30A so ...

Their data sheets do specifically call for 0.25 cal/cm? incident energy with 1.2 cal/cm? PPE required. That was fortunately not an eye-test for me.

I picked the small size specifically because that's what we use on our buss runs to supply our smaller machine tools. So it's a bit more familiar to me and a little more important for me to understand. I figured it actually exceeded the discussion since I don't know too many homes with drops from 400A 480V 3-phase 75kA (Bolted Fault) service. I'm not into that high a rent district yet

They're showing the same incident energy and boundary for the 400A fuse and the eye-ball test is about 2300A for the current-limiting to kick in. I'd be interested in seeing that 30' analysis that kicks it up to an 8 cal/cm? PPE. Arc flash is relatively new at our site and our official EE for the buss isn't worth consulting.

 

[jim dungar]

... 75kA (Bolted Fault) service.....

The problem is when the fault current is not very high. When you enter the area of typical branch circuits, low fault currents are not uncommon.

At 2.3kA a LPS-400RK will not reduce the arc flash hazard below 8 cal/cm?, in fact it could take more than 7 secs to clear this low of a fault. By the same token a fault current of 3.5kA will yield a maximum of 1.2 cal/cm?. This fuse enters its current limiting region around 9kA.

 

[pfalcon]

The problem is when the fault current is not very high. When you enter the area of typical branch circuits, low fault currents are not uncommon.

At 2.3kA a LPS-400RK will not reduce the arc flash hazard below 8 cal/cm?, in fact it could take more than 7 secs to clear this low of a fault. By the same token a fault current of 3.5kA will yield a maximum of 1.2 cal/cm?. This fuse enters its current limiting region around 9kA.

Oops, Eye-chart failure. LPS-RK-400SP Bussmann goes current limiting somewhere between 6kA and 7kA. The LPS-RK-200SP is around 2300A.

 

[zog]

The problem is when the fault current is not very high. When you enter the area of typical branch circuits, low fault currents are not uncommon..

And an arcing fault current is even lower, as low as 35% of bolted fault current in a 480V system. This is important because we are talking about arc flash here, arc flash reduction was never the intent of current limiting fuses and I think they get marketed as an arc flash solution when in fact they often are not. I have seen many industrial plants just replace fuses with current limiting ones and slap HRC 0 on all of thier panels. Bussman does free arc flash training for a reason