When an arc flash hits the outer layer of FR clothing, the garment heats up tremendously. With enough incident energy, heat can be transferred through arc-rated clothing and melt the under-layers causing painful burns. Under-layers, as the name implies, is the clothing worn under the arc flash protective clothing.
It has been recognized for some time that wearing layers of clothing will increase the ability of the PPE system to dissipate the heat from an arc flash, but you should know that simply putting an 8 cal coverall over a 4 cal shirt doesn’t necessarily give you a 12 cal system.
Layers of arc-rated garments may provide additional protection if the individual layers have been tested together as a system to be used as more than the highest individual layer. Testing is done per ASTM F1959 to verify compatibility and establish performance data.
The word “system” must be emphasized because PPE and arc-rated clothing should be viewed as a system of individual articles of clothing and equipment that, when used together properly, greatly improve their performance over that when used individually.
Cotton under-layers are recommended in most circumstances, although it’s important to understand that cotton undergarments may add some protection but may also ignite, causing serious burns. Non arc-rated clothing may be worn IF it is non-melting (i.e. cotton, wool, silk, leather).
Wearing arc rated under-layers, even if you do not know the added protection level, eliminates the risk of ignition and has great value in work settings where high voltages are present and where arc flash levels are higher than 15 cal/cm².
Non arc rated melting vests may not be worn over arc rated layers. A non arc-rated outer layer (such as a high visibility vest) over a protective system (an 8 cal/cm² ARC 2 shirt, for example) does not reduce the rating of the under layer.
Don’t discount the power of layering. It is the most reasonable way to protect workers and still manage heat stress and costs. Just be sure to avoid the short cuts of “adding up the layers.”