There’s a few points to address when answering this double-barrelled question. Starting with the term ‘dielectric’…
According to various sources, dielectric refers to a material which is a poor conductor of electricity.
Moving on, when is a boot an over boot? Very simply, the over boot can be worn over existing footwear. In a vehicle repair or maintenance workshop environment you might expect staff to be wearing protective footwear. The specification of such footwear might include toe protection and anti-slip soles.
The dielectric over boots provide the additional protection of insulation. This is in the event that the technician comes into contact with a live electric source. In such an event, the over boot could prevent electric shock.
Being an over boot means that it’s not necessary to remove existing safety footwear. This makes the over boot very convenient. Given that it’s unlikely the technician would need such protection, the over boot can be quickly fitted and worn on demand.
For workshops carrying out maintenance or repair of electric hybrid vehicles, such protective clothing is essential to protect workers from the high voltages stored in vehicle batteries.
OSHA PPE General Guide
According to 29 CFR 1910.136(a): “Each affected employee shall wear protective footwear when working in areas where there is a danger of foot injuries due to falling or rolling objects, or objects piercing the sole, and where such employee’s feet are exposed to electrical hazards.” Appendix B of Subpart I identifies the following occupations for which foot protection should be routinely considered: shipping and receiving clerks, stock clerks, carpenters, electricians, machinists, mechanics and repairers, plumbers, assemblers, drywall installers and lathers, packers, wrappers, craters, punch and stamping press operators, sawyers, welders, laborers, freight handlers, gardeners and grounds keepers, timber cutting and logging workers, stock handlers and warehouse laborers. http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=10120
While in the UK the Health and Safety Executive provide general guidelines on electric and hybrid vehicles, they offer no specific guidance on protective foot wear.
OSHA 29 CFR 1910.269, which applies to the transmission, distribution, and generation of electricity, cites ASTM F1117 shoes in the standards document but gives no guidelines as to when they are needed. http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_id=9873&p_table=STANDARDS
OSHA gives little guidance and really mentions the EH shoes only in the general PPE guide for small businesses. OSHA states, “Electrical hazard, safety-toe shoes are nonconductive and will prevent the wearers’ feet from completing an electrical circuit to the ground. These shoes can protect against open circuits of up to 600 volts in dry conditions and should be used in conjunction with other insulating equipment and additional precautions to reduce the risk of a worker becoming a path for hazardous electrical energy. The insulating protection of electrical hazard, safety-toe shoes may be compromised if the shoes become wet, the soles are worn through, metal particles become embedded in the sole or heel, or workers touch conductive, grounded items. Note: Nonconductive footwear must not be used in explosive or hazardous locations.” http://www.osha.gov/Publications/osha3151.pdf