Lock out Tag out verification

[don_resqcapt19]

...
On a side note your responses raise a couple of interesting questions:

1. Touch voltages on 230VAC systems are a big deal in Australian systems requiring every circuit to have its earth fault loop situation considered. Is that different to NFPA requirements?
2. Installing plug/sockets on cables in Australia requires the use of flexible cables rather than building wiring cables. Specifically, greater strand count and more flexible insulation and sheathing. Is that different to NFPA requirements?

Where does the touch voltage come from after the equipment has been disconnected from its power source? The NEC has no requirements to look at the earth fault loop. The NEC is a prescriptive document and specifies how to install the required bonding and grounding.

Typically the motor would be fed using stranded conductors and liquid tight flexible metal conduit. Adding the Meltric at the motor would not change this. While you can use cables with the Meltric connectors, nothing in the NEC would require you to. If the Meltric is being installed just to provide a local disconnect, it would be my opinion that the NEC would prohibit the use of flexible cable. Our used of flexible cable is very limited.

 

[cdevine]

Where does the touch voltage come from after the equipment has been disconnected from its power source? The NEC has no requirements to look at the earth fault loop. The NEC is a prescriptive document and specifies how to install the required bonding and grounding.

Typically the motor would be fed using stranded conductors and liquid tight flexible metal conduit. Adding the Meltric at the motor would not change this. While you can use cables with the Meltric connectors, nothing in the NEC would require you to. If the Meltric is being installed just to provide a local disconnect, it would be my opinion that the NEC would prohibit the use of flexible cable. Our used of flexible cable is very limited.

Yes the touch voltages come from other energised, faulted equipment and it uses the conductive process elements eg pipes together with their electrical earth connections to return earth fault loop currents to the source. Having many hardwired devices that incorporate permanent earth connections just gives you extra protection through the multiple paths. This contribution cannot be incorporated in designs but is significant practically in 230V systems particularly in high touch voltage risk areas eg, wet, heavy industry environments.

Thanks for the heads up on earth fault loop and flexible cable requirements.

 

[don_resqcapt19]

Yes the touch voltages come from other energised, faulted equipment and it uses the conductive process elements eg pipes together with their electrical earth connections to return earth fault loop currents to the source. Having many hardwired devices that incorporate permanent earth connections just gives you extra protection through the multiple paths. This contribution cannot be incorporated in designs but is significant practically in 230V systems particularly in high touch voltage risk areas eg, wet, heavy industry environments.

Thanks for the heads up on earth fault loop and flexible cable requirements.

That is kind of what I had expected, but with all of the non-electrical metal energized, I don't see any additional hazard with the removal of one Equipment Grounding Conductor. The intent of the NEC is that the EGC for the faulted equipment clear the fault, of course the metal parts of the faulted equipment will be energized with a voltage equal to the voltage drop on the fault return path(s) until the fault is cleared. This will also energized other conductive items that are physically connected the faulted equipment. This will happen even if I have not disconnected the EGC for the motor that is being worked on. Given all of the paths in a typical industrial application, I would not expect that the removal of one EGC from the paths would make any significant difference in the touch voltage if there were to be a fault on other equipment while I have this motor disconnected.

 

[cdevine]

That is kind of what I had expected, but with all of the non-electrical metal energized, I don't see any additional hazard with the removal of one Equipment Grounding Conductor. The intent of the NEC is that the EGC for the faulted equipment clear the fault, of course the metal parts of the faulted equipment will be energized with a voltage equal to the voltage drop on the fault return path(s) until the fault is cleared. This will also energized other conductive items that are physically connected the faulted equipment. This will happen even if I have not disconnected the EGC for the motor that is being worked on. Given all of the paths in a typical industrial application, I would not expect that the removal of one EGC from the paths would make any significant difference in the touch voltage if there were to be a fault on other equipment while I have this motor disconnected.

Ah yes but what happens on a maintenance down day when multiple items are unplugged? Each one you isolate removes another return path. In the industries where I work group isolations on shutdown days are very common. I've got a feeling that this is nothing new to you either.

You are right one motor is not a big deal and to be honest this issue alone is not a big deal but its just one problem of many (that is not immediately obvious) why plug and sockets may not gain wide spread application.

 

[don_resqcapt19]

Ah yes but what happens on a maintenance down day when multiple items are unplugged? Each one you isolate removes another return path. In the industries where I work group isolations on shutdown days are very common. I've got a feeling that this is nothing new to you either.

You are right one motor is not a big deal and to be honest this issue alone is not a big deal but its just one problem of many (that is not immediately obvious) why plug and sockets may not gain wide spread application.

I just don't see it as big deal...the code required fault return path should cause enough current to flow that the touch potential only exists for a very short period of time, and the touch potential at the remote point would never exceed the one at the faulted equipment.

 

[cdevine]

I just don't see it as big deal...the code required fault return path should cause enough current to flow that the touch potential only exists for a very short period of time, and the touch potential at the remote point would never exceed the one at the faulted equipment.

As I confirmed, it is not a big issue in isolation.

But just to respond to your last point, the equipment that has been unplugged (not directly connected to the power system earth) is being maintained by a guy standing in a puddle of water. An earth fault occurs on a remote motor and a portion of the earth fault current returns via the connected pipework such that an earth potential rise exists between the guy's hands and his feet. It may not be much in magnitude and it may be short in duration but it may not be either. It could be a faulted HV motor protected by motor start fuses. The point is that if a plug and socket was not used the equipment being maintained may have a better earth through its direct connection and therefore reduced touch voltage. Now aside from this unlikely but not impossible event described there is still the issues of:

• People contact with cables
• Damage to plugs and sockets when dropped due to their weight and that of their connected cable. I have seen plenty of polymeric plugs broken but I agree they are tougher than brittle die cast aluminium.
• Limited current ranges (approx 200A)
• If you don't put the cap on, introduced foreign bodies into coupling eg water, non conductive dusts
• Manual handling issues due to weight of plugs, sockets and connected cable and having to overcome spring pressure on connection
• Plug/Socket high cost
• Two extra terminations per motor yielding high cost

Am still not convinced Don. I am enjoying the exchange though!

 

[don_resqcapt19]

All of what you say is correct, but if you want 100% assurance that the power has been disconnected from the motor, there is absolutely no question when you unplug it.

The next best would probably be a local disconnect with a viewing window in the disconnect so you can see the status of the knife blades. I have seen Canadian equipment with this type of disconnect. Not sure if their code requires the viewing window. Also probably not available for use in classified areas.

However local disconnects are not very common in the industrial plants in my area. Most equipment is fed from a remote MCC without a local disconnect per the exception to 430.102 in the NEC.

Also they do make connectors up to 600 amps, but they are not load break rated and they are very heavy.

 

[cdevine]

All of what you say is correct, but if you want 100% assurance that the power has been disconnected from the motor, there is absolutely no question when you unplug it.

The next best would probably be a local disconnect with a viewing window in the disconnect so you can see the status of the knife blades. I have seen Canadian equipment with this type of disconnect. Not sure if their code requires the viewing window. Also probably not available for use in classified areas.

However local disconnects are not very common in the industrial plants in my area. Most equipment is fed from a remote MCC without a local disconnect per the exception to 430.102 in the NEC.

Also they do make connectors up to 600 amps, but they are not load break rated and they are very heavy.

Yes I agree unplugging provides the best assurance of isolation. However, in too many cases it is impractical and costly therefore it may be difficult to regard it as sensible solution let alone a universal solution.

Visible break isolators have been popular in recent times particularly in mining applications. Again they seem to offer a simple form of isolation as well as being local to the equipment. However, they have their shortcomings as well. So much so that we know of several sites that have deployed VBIs who are now looking for an alternative. Problems cited include:

• Difficult to determine whether the disconnect is open - yes that's right the very problem that they claim to address. It can be quite unclear even for electricians what to actually look at inside the disconnect. What chance does an operator have?
• Chemical / pollution causing glass opaqueness
• Hard to see at night
• Limited load break / fault make capabilities compared with conventional isolation switches
• Very high capital cost
• Two extra terminations per motor yielding high cost

The arrangement "equipment is fed from a remote MCC without a local disconnect" is very common in our neighborhood as well.

The more I see of the problem of isolation verification for mechanical works the more I question whether regulators have an understanding of the impracticality of existing approaches. Your Try-Lock-Try approach looks OK so long as the infrastructure is already in place. Yes it still is not the best for reasons discussed earlier meaning some people that have had to decide recently whether to implement the Try Start method and spend money installing Try Start stations everywhere have chosen alternatives.

I find that a lot of people tend to fall back on a Live-Dead-Live check even for mechanical works. I guess they regard it as a high integrity result. After a while they realise that the productivity of their electricians is very poor. That was the motivation for the development of a device that allows a non-electrician to do a Live-Dead-Live style check.

 

[don_resqcapt19]

... Your Try-Lock-Try approach looks OK so long as the infrastructure is already in place. Yes it still is not the best for reasons discussed earlier meaning some people that have had to decide recently whether to implement the Try Start method and spend money installing Try Start stations everywhere have chosen alternatives. ....

Yes, it would be very costly to install if you are adding it to an existing equipment.
And the biggest issue is you can't use it if the equipment is not working and equipment repair is a major reason to do a lockout. Here it is mostly used for equipment that is associated with the equipment that is to be repaired and for routine maintenance on equipment that is working. It is also used for the initial start-up testing on newly installed equipment for things like motor rotation and mechanical interference.

 

[cornbread]

Great discussion. I'm working with our safety dept. and hopefully we can come up with a system that works. I hear the argument about it being costly, but on accident will offset any cost.