This is a question we get fairly often, and for good reason: it’s not uncommon to wear an undershirt off the clock, for added comfort or warmth. How does this common practice apply in an energized work environment? What do the standards say, and how do the remaining options rate in the event of an arc flash? We recently put these questions to the test by conducting LIVE arc flashes at an independent laboratory in Chalfont, PA – resulting in excellent video of the consequences.
Common industry standards do not require a base layer, so long as the outer layer is FR and matched to the hazard. However, industry standards do say that any base layers cannot be meltable. Watch this arc flash demonstration to see why – the manikinin this arc is wearing a meltable base layer under a 7oz FR 88% cotton / 12% nylon shirt with an arc rating of 8.6 cal:
As we can see, the base layer ignites and melts. The fire goes out fairly quickly because the base layer has melted, limiting the fire spread, but leading to an equally serious problem: molten polymer all over the torso.
The arc-rated shirt would have protected the wearer from any second-degree burn in this exposure, because the rating of the shirt was higher than the incident energy of the arc. However, the base layer igniting briefly and melting has dramatically changed the outcome – from no injury to serious burns. Burns caused by liquids like molten polymer are most often third degree, which requires skin grafting.
Eliminating meltable base layers leaves you with three options: wear no base layer, wear a 100% cotton base layer, or wear an arc-rated (AR) or flame resistant (FR) base layer. Wearing no base layer allows you to comply with the standard, while neither detracting from nor adding to the protection afforded by the FR outer layer – so long as your FR outer layer is worn properly, with tails tucked in, cuffs down and buttoned, and the shirtfront buttoned to the top. How does that compare with option 2 – a non-FR but non-meltable cotton base layer – or option 3 – an AR/FR base layer?
In this next exposure, the worker is wearing a 100% cotton base layer under a 6oz FR 88% cotton / 12% nylon sateen shirt with an arc rating of 9.5 cal. However, the AR/FR outer layer is worn improperly – it is not tucked in, allowing the arc to get underneath and ignite the cotton tee shirt:
In this arc, we can clearly see a T-shirt fire at the very top – causing serious facial injury and likely respiratory damage. Later, when we rip the outer FR shirt open to expose the cotton undershirt below, we see the damage caused by the T-shirt fire burning underneath the FR layer.
This video is also an excellent example of why it’s critically important to wear FR properly – with shirt tucked in and buttoned up. As we have seen, although the 100% cotton base layer complies with the standards because isn’t meltable, it is in fact flammable.
By choosing to wear a flammable base layer, the worker is left open to injury in the event of an arc flash – by making simple mistakes like forgetting to tuck in the AR/FR shirt, forgetting to button the top button, or allowing the cuff of the cotton undershirt to sneak out from under the sleeve of the FR outer layer. Additionally, non-FR base layers do not count toward system arc ratings, while AR/FR base layers do.
Now, let’s compare this outcome to one in which the worker was wearing an arc-rated base layer. In this exposure, the worker is wearing the FR base layer as a standalone layer for demonstration purposes in what is “only” a CAT 1 arc:
Despite the significance of the arc, the base layer does its job, saving the wearer from injury. Imagine the difference in outcome if the wearer in our second video had worn this base layer instead of the 100% cotton one.
As we have seen, a cotton undershirt is better – in terms of both compliance and protection – than wearing a meltable one, but it – like wearing no base layer at all – leaves the worker open to injury in the event of human error, like an undone button on the AR/FR outer layer. An arc rated base layer is not required but is best practice – providing both an added layer of protection and peace of mind.
Which option would you choose if your life were on the line?