An Acceptable Method for Live >50V Work?

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HumbleMan

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Looking for some opinions on the following scenario.

I sometimes have to troubleshoot controls on older equipment where the control voltage is >50V, typically 120VAC on 480V powered units. Our location's live work policy meets 70E, no live work unless it's approved by the plant manager and you filed the appropriate paperwork and he usually says "no".
The troubleshooting iteration typically is to power off, LOTO, add probes to likely points to locate the problem, close the door, re-energize and see if you guessed correctly where to place the probe points.

Per the standard:
NFPA 70E Article 130.1 Justification for work. Live parts to which an employee might be exposed shall be put into an electrically safe work condition before an employee works on or near them, unless the employer can demonstrate that deenergizing introduces additional or increased hazards or is infeasible due to equipment design or operational limitations.

What if:
1) The main power remains under LOTO so there is no chance for Shock, Flash, or Boom.
2) The control power supply is disconnected from the stepdown transformer where it normally supplies the 120VAC. (No backfeeding to the primary of the transformer.)
3) A separate source of 120VAC is used and run through a GFCI outlet or breaker and is then used to power the 120VAC control circuit.

Since a GFCI by its nature limits current flow to around 5mA within 25 ms prior to tripping, wouldn't this "put into an electrically safe work condition" the equipment so that the control functionality could be tested live?

Additionally, do you think a "Live Work Request" needs to be filed each time this is done?
 
Troubleshooting and voltage testing does not require a hot work permit per NFPA 70E.

130.1(B)(3) permits troublshooting and voltage test to be performed without needing an energized electrical work permit, provided appropriate safe work pratices and personal protective equipment in accordance with Chapter 1 are provided and used.

Chris
 
HumbleMan said:
What if:

1) The main power remains under LOTO so there is no chance for Shock, Flash, or Boom.

2) The control power supply is disconnected from the stepdown transformer where it normally supplies the 120VAC. (No backfeeding to the primary of the transformer.)

3) A separate source of 120VAC is used and run through a GFCI outlet or breaker and is then used to power the 120VAC control circuit.

Since a GFCI by its nature limits current flow to around 5mA within 25 ms prior to tripping, wouldn't this "put into an electrically safe work condition" the equipment so that the control functionality could be tested live?
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No.

GFCIs do not limit current or voltage at all, they simply open when the current threshold is exceed, by that time you are already getting a full voltage and full current shock until the circuit opens.
 
GFCIs do not limit current or voltage at all, they simply open when the current threshold is exceed, by that time you are already getting a full voltage and full current shock until the circuit opens.
Click to expand...

Excuse my ignorance but from what I have read, ventricular fibrillation normally requires currents of at least 60 mA. If it has a direct pathway through the heart, it can be as low as 1 mA.

If you are wearing appropriate PPE including approved gloves, wouldn't this minimize shock unless you accidently somehow became a conductor to ground?
 
HumbleMan said:
Excuse my ignorance but from what I have read, ventricular fibrillation normally requires currents of at least 60 mA. If it has a direct pathway through the heart, it can be as low as 1 mA.
Click to expand...

Again the GFCI has no ability to limit the current.

Imagine you are getting a shock from hot to ground a on a GFCI protected circuit. For the time it takes the sensor to react to this sudden spike in current and the time it takes the contacts to open you will be subject to whatever voltage and current the circuit is capable of providing.

If you want dramatic evidence of this take the hot of a GFCI protected circuit and touch it to the EGC. You will see a big spark and missing metal before the GFCI has a chance to open.

But regardless of all that, OHSA does not recognize a GFCI as make a circuit safe to work on.

OSHA

1910.333(a)(1)

"Deenergized parts." Live parts to which an employee may be exposed shall be deenergized before the employee works on or near them, .........
Click to expand...

It goes on but the jest of it is this, either the circuit is dead, or it is live but you are troubleshooting with proper PPE.

Now that the plant manger has stopped that you are out of luck.



If you are wearing appropriate PPE including approved gloves, wouldn't this minimize shock unless you accidently somehow became a conductor to ground?
Click to expand...

Maybe, I don't know ........ but I do know it is not a recognized way to meet the OSHA requirements.
 
Your GFCI number seems pretty optimistic but I think a GFCI does cause an electrically safe work condition for almost everyone by limiting the energy dumped into the person shocked.

Here is the formula for the GFCI trip curve according to UL943. T in seconds and I in mA are maximums.
T=(20/I)^1.43

5 >enter I in mA
7 =calc'd trip time in seconds
4.4 =calc'd joules dumped into "load"

100 >enter I in mA
0.1 =calc'd trip time in seconds
1.2 =calc'd joules dumped into "load"

400 >enter I in mA
0.014 =calc'd trip time in seconds
0.7 =calc'd joules dumped into "load"

I picked the 400 mA because a 2400 VAC elec. chair puts 8A max into the subject so 120/2400 x 8 A = 400 mA.
You'll also notice that this trip time corresponds to the probable minimum trip time for a mechanical relay.

Also, above 1 joule can be fatal but these defib machines can dump 200 joules into somebody.
 
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