A shoe is half of a system, the floor is the other half
Two ways feet reach ground
Standing operators are grounded through footwear in one of two ways. ESD shoes have conductive or dissipative soles that couple to an ESD floor directly. Ordinary shoes plus a heel or toe strap work differently: the strap bridges from the wearer's skin over the shoe to the floor, so charge on the body has a path out.
In both cases the floor is doing half the work. Shoes or straps on a waxed, insulative or uncontrolled floor provide almost nothing, which is why footwear is specified as a footwear-flooring system rather than as a product on its own.
The failure modes that hide in plain sight
Heel straps worn over socks rather than against skin are the classic one, because the strap looks correct from a distance while the conductive path ends at the sock. Other quiet failures include soles caked with dust or production residue, straps on the wrong foot, and floor surfaces contaminated or coated in a way that insulates them.
Dry environments add a subtler problem: skin contact resistance rises, and a system that measured fine in humid summer can drift out of range in winter. None of these are visible at a glance, which is why footwear is tested rather than inspected.
Verifying the path on the floor
The basic check is a resistance measurement through the wearer at a footwear tester, typically at the entry to the EPA. Walking tests take it further, measuring the charge generated as the person actually moves through the area, which catches combinations that pass a static resistance test but still generate high voltages in use.
Set the frequency to match risk and turnover: a common pattern is a test at each entry for operators in footwear-only areas, plus periodic verification of the floor system itself. Record failures and fix causes rather than re-testing until the number passes.
FAQ
Do heel straps work over socks?
Generally no. The strap must contact skin to drain charge from the body. If sock coverage is unavoidable, look at conductive sock options or a different grounding method, and verify with a tester rather than assuming.
Can ESD shoes replace a wrist strap?
For standing work on a suitable ESD floor, footwear is often the primary grounding method. Seated operators usually still need a wrist strap, since chair contact and posture make footwear unreliable at a bench.
How often should footwear be tested?
A frequent pattern is a check at EPA entry using a footwear tester, with the interval and limits defined in the site plan. What matters is that the test is part of the routine, not occasional.
Why does the floor matter so much?
Footwear drains charge to whatever is under it. On an insulative or contaminated floor there is nowhere for the charge to go, so even certified shoes leave the operator ungrounded.
All figures above refer to standard product data sheets; final confirmation is subject to written quotation.