Shock Complaints at Drive-Through Cash Machines, Florida (2018)

In 2018 an ATM manufacturer asked us to investigate reports from Florida that customers at drive-through machines felt a shock when they touched the PIN pad. We examined the units that had been reported. The client and the owner of the machines are not named.

Looking for a hazard first

A shock complaint has to be treated as a possible hazard until testing shows otherwise. We tested the grounding of the machine at eighteen points and at both outer doors, with the unit powered off and powered on, measuring resistance in both directions along with AC and DC millivolts between points. Every path passed. With power off, resistances were 0.2 ohm or less, the ground was isolated from both the hot and the neutral, and the highest leakage reading was 0.5 millivolts DC. The OSHA rules on guarding live parts start at 50 volts, about 100,000 times higher. Leakage at that level cannot be felt.

The likely source

The customers were sitting in their cars. Sliding across a cloth seat builds a static charge, and the charge jumps to the first metal at a different potential that the person touches. On this machine, the frame around the PIN pad was grounded metal. A static discharge to it can be startling and uncomfortable, but static from clothing and car seats is generally harmless. We recommended replacing the metal frame with an insulating one. A ground rod at the machine can help equalize charge, but it would not stop a nuisance shock to a driver who has not discharged before reaching for the keypad. In no case should a machine be installed without a code compliant ground back to the building that feeds it.

Design notes the client asked for

The client also asked us to review the power supply, a question separate from the shocks. Internal components were grounded in a daisy chain instead of to a single bus. The internal wiring was #12 AWG, which the NEC limits to a 20 A protective device, on a supply marked for a 30 A circuit, so we recommended #10. The 120 V receptacles on the distribution block would see 240 V if the unit were connected to 240 V, so we recommended a selector that de-energizes 120 V parts in that case. Other items included putting the heaters and air conditioner on their own circuit, guarding the live input terminals, adding a wiring diagram at the connection point, not using the surge suppressor as a terminal for several wires, and correcting reversed wire colors on the DC converter.

Reports of shock

Many shock complaints turn out to have an ordinary explanation, but the measurements have to come first. Our electrical injury and shock work starts the same way every time, whatever the source turns out to be.