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 Post subject: Releasing Stored Electrical Energy
PostPosted: Fri Dec 01, 2017 7:56 am 
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2018 NFPA-70E Article 120.5 lists (8) steps for Establishing and Verifying an Electrically Safe Work Condition. It also states that the steps shall be performed in the order presented, if feasible.

Step 2 is to interrupt and disconnect each supply of electricity.
Step 4 is to "Release stored electrical energy."
Step 6 is to "Apply lockout/tagout devices..."
Step 8, part 1, is to "ground the phase conductors or circuit parts before touching them", if "stored electrical energy exists".

It appears to me that there is a conflict here.

If you were locking out a circuit that contained a capacitor, would you discharge the capacitor BEFORE or AFTER you hung the lock?

What kind(s) of stored electrical energy should be released BEFORE hanging a lock?


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 Post subject: Re: Releasing Stored Electrical Energy
PostPosted: Sat Dec 02, 2017 9:31 am 
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There is a larger problem here. OSHA 1910.269 has the order one way and OSHA Subchapter S has it the other way...so they can't agree either.

Here are the pros and cons of the ordering and why I believe that it matters but I haven't really come up with a good solution:
1. What if a mistake is made in identifying the correct circuit to be locked out? If testing for absence of voltage is performed first, it can avoid the obvious problem that performing grounding of any kind first will create a dead short and most likely an arc flash. When induced or stored charges are not present or at least not at "full voltage", this condition can be safely detected and eliminated without causing an arcing fault.
2. Where induced voltage or stored charge are present, testing for absence of voltage before discharging them is a complete waste of time since it will simply confirm that voltages are present and the subsequent steps may or may not have cleared the voltages which is the whole reason for the test. Granted it may be possible to tell the difference in both cases if the measurement tools actually show a value rather than just an idiot light as a tic does, but particularly at medium voltage where the problem usually exists, it may not matter.

A related but similar issue is HOW to discharge. With induced voltages, the argument about equipotential planes vs. "working between the grounds" has raged on for several years and getting past the first round of pro/con arguments, there are again situations where one is more beneficial than the other depending on the conditions and so far other than OSHA's requirement to work in an equipotential work zone, there is no definitive argument either way.

Still yet another problem and maybe this points to a solution is how to discharge capacitors safely. It is less of an issue for induced voltages which often have very little power so an arc doesn't pose much of a hazard. The best approach to discharging capacitors safely is to do so it in a controlled manner which means using a resistor to control current flow to a safe level, which also prevents dangerous arcing faults as a result. Of course there aren't "discharge sticks" on the market...you have to make one. But it does point to a solution. And this doesn't stop the situation where induced voltages (static or inductive) may be present and this is best solved with a traditional ground cluster.

Another related problem has to do with equipment design with regards to the doors. A lot of rotary disconnects, particularly the really bad designs that GE uses, cannot be opened or closed once the lock is in place. So the door has to be either locked opened or closed because the lock can't pass through the door. This would seem to support the NFPA 79 requirement that only the lever handle style disconnect handles are acceptable (and I vastly prefer them myself) but a few of these are mounted so close to the door lip that again the lock gets in the way making it almost a situation where the door can only be locked open or closed. This is less of a problem when electricians are locking out doors but it is particularly a problem with multiple work crews where mechanics and production personnel have no interest or need to open the door so they tend to lock the door shut making electrical tasks and shared lockouts impossible.

So....I'm not really satisfied with 70E's solution but with simple rules and no consideration for other potential issues, it's not really as cut and dry as it would seem.


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 Post subject: Re: Releasing Stored Electrical Energy
PostPosted: Mon Dec 04, 2017 7:23 am 
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Medium and High voltage cables are shielded. The shield acts like a capacitor and will store charge.. Once you open the circuit, you check with a tic-tracer to show no active AC voltage, then you must use a grounding stick to discharge the store energy. Perhaps this is what they are referring to. With capacitor, they typically have a discharge resistor built in, so the trick there is to wait 10 minutes or so so that the capacitor can discharge slowly through its resistor. Capacitor enclosures have grounding throw over bars, but hell of a snap if you do it right after opening the circuit.


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 Post subject: Re: Releasing Stored Electrical Energy
PostPosted: Tue Dec 05, 2017 9:33 am 
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I'll give you another example to chew on: electrostatic precipitators. The way they work is to put medium-high voltage (typically 50 kV+) onto some kind of grid setup that attracts particles such as soot and dust in an air stream. The thing pulls milliamps of current on the high voltage side. It won't work correctly if it grounds and I have yet to see one with a bleed resistor other than whatever natural bleeding occurs. And just like a capacitor there is a massive amount of surface area and a huge amount of stored charge in these things.

A tic by the way only shows AC voltage fields. It does NOT detect DC stored charge. To meet OSHA and 70E standards if you have DC and we're dealing with high voltages (above 1000 V) the the multimeter is useless and the tic tracer doesn't work either. That's where some other kind of meter becomes necessary., if we are to meet the requirement of testing for absence of DC voltage. Otherwise this is exactly like ignoring the tic and just throwing a ground cluster on it and watching the fireworks as either (1) the overcurrent protection trips or (2) stored charges are discharged. So if you know you have DC present then aren't you supposed to measure it?


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