There was a time when FR (flame resistant) clothing was issued, and then terminology referencing personal protective equipment (PPE) was changed in 2012 by NFPA. The NFPA – National Fire Protection Association, is a leading information and resource for fire, electrical and related hazards. https://www.nfpa.org/

What had previously been referred to as FR (flame resistant) clothing is now called Arc Rated or AR.
The reason for the change is that all clothing with an arc rating (AR) is flame resistant (FR) but not all FR clothing has an Arc Rating. FR doesn’t have any standards applying to any particular hazard.

For full body protection, the protective clothing shall be worn in the closed state and other suitable protective equipment (e.g. helmet with protective face screen, protective gloves and footwear (boots) shall be used. No garments such as shirts, undergarments or underwear should be used which melt under Arc exposures – e.g. made of polyamide, polyester or acryl fibres. For maximum protection against Arc Flash Exposure garments should be worn as part of a layering system i.e. in conjunction with a shirt or other undergarment.

IEC 61482-2 Standard Specification for Thermal Effects of an Electrical Arc Event
About this Specification
IEC 61482-2 is the European equivalent to ASTM F1506 for the thermal hazards of an arc flash. The standard includes requirements for material testing and additional information for garments constructed from compliant materials. This is required for selling garments for arc flash protection in the EU.

BS EN 61482-1-2:2014
Live working. Protective clothing against the thermal hazards of an electric arc. Test methods. Method 2: Determination of arc protection class of material and clothing by using a constrained and directed arc (box test)
Status : Current Published : January 2015

 

According to the arc rating standards (ASTM F1506, ASTM F1959, IEC 61482-2, IEC 61482-1-1 Method A and IEC 61482-1-1 Method B), it is a prerequisite that clothing flame resistant (FR) before it can even be tested to define its arc rating.
It is crucial that you read the labels of garments and equipment before buying and to specify a standard to meet. If an arc flash hazard is apparent, the AR label should be present on any PPE. Choosing a garment labelled FR will not be sufficient protection in an arc flash situation.