jtinge wrote:
Consider the following:
A non-metallic control box containing cord-connected vibration measurement devices for a motor. The 120 volt power terminals for the devices have a lexan or plastic guard to prevent inadvertent contact. The remaining terminals and conductors operate at less than 50 volts. The equipment in the box can be de-energized by unplugging from a local 120 volt outlet.
We do not perform arc-flash hazard analysis on cord-connected equipment fed by 20 amp branch circuits.
For the equipment and configuration described above, what shock hazard and arc-flash hazard PPE is required for qualified workers to do work only on the less than 50 volt terminals and conductors if the equipment is energized?
Shock hazard: That depends. If inadvertent contact with the 120 V conductors is possible because they are NOT guarded, insulated, or isolated, then the shock hazard table for 120 V clearly indicates that contact must be avoided. So if the goal is to manipulate exposed, energized conductors with either non-insulated tools or bare hands while energized (or more specifically, not electrically locked out/disconnected) and there are exposed conductors near enough that they pose a hazard to inadvertent contact then rubber gloves and/or leather protectors are necessary. Note that the exact same requirements apply to the 120 V conductors themselves but since you've got a "mixed voltage" environment we can't just ignore the other conductors that are present.
Arc flash hazard: This is where 70E breaks down by suggesting that you need to somehow figure the arc flash hazard where basically no standard applies. Applying the "rule" from IEEE 1584, we can ignore hazards for equipment fed by a transformer (or equivalent power source) for 208 V, 125 kVA, 3 phase, or below. So no arc flash PPE needed.
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Does the lexan cover over the 120 volt line terminals allow them to be considered suitably guarded such that they would not be considered exposed as described in NFPA 70E 2015 definitions for Exposed and Guarded below?
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Does it? Under 70E, this is a judgment call. There is not standard beyond the verbiage in 70E but the closest standard that anyone is using is the IEC "finger safe" standard which is definitely overkill for defining "guarded" but it is a conservative standard if you are having trouble making a judgement call on this.
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Does this equipment need to be de-energized by unplugging from the outlet to work on the less than 50 volt terminals and conductors?
Let's consider the shock hazard. We can reference Dalziel's work for this. Basically at around 100 mA or above there is a significant likelihood of causing fibrillation of the heart. The threshold resistance of the body is debatable and specifically IEC gives different numbers that are voltage dependent but under say IEEE Standard 80 we're looking at a 1 K resistance. 50 V / 1,000 ohms = 50 mA. This is high enough that certainly it is above the thresholds for startling someone and getting into the pain threshold but as far as fibrillation goes, it is not likely. Further there has never been a case in OSHA's records of a fatality at 50 V or less. And as further evidence the whole telecommunications industry uses 48 VAC as a standard working voltage and they are pretty darned cavalier about handling energized conductors so if they haven't had any medical issues, it probably won't happen.
As far as the arc flash hazard goes some testing was done a few years ago at Kinetrics at 130 VDC (not even AC) where they testing for substation batteries (125 VDC) at up to 20 kA with very short electrode distances. As we decrease the voltage the distance requirement becomes even more critical...almost direct contact (scratch starting) becomes necessary for arc welders without HF or "pulse" starting for instance. Arcs and arc flashes at less than 130 VDC (or AC) seem to be at least possible but little information exists because nearly all the time even under laboratory conditions the arcs self-extinguish. Even the IEEE 1584 data set has only a single condition where they achieved a stable arc at 208 VAC. The current empirical model in IEEE 1584 realistically cannot be extended outside of that single data point because no arc was achieved at any other condition. Hence the reason that IEEE 1584 states that under the cutoff conditions (208 V or less, 125 kVA or less source), arc flash can be ignored.
