Static Electricity 2000 Edition

A.8.10
See Britton, Avoiding Static Ignition Hazards in Chemical
Operations, for additional information.
A.8.11
See Britton, Avoiding Static Ignition Hazards in Chemical
Operations, for additional information.
A.9.3.2
See NFPA 33, Standard for Spray Application Using Flam-
mable or Combustible Materials, for further information.
A.9.4.2.1
Investigations into the static electric hazard in grain
elevators have shown that danger exists when the voltage
reaches 30,000 volts on the belt. These studies also show that

---

APPENDIX B
77
–41
2000 Edition
low relative humidity is an important factor in that it allows volt-
ages to increase very rapidly at temperatures below freezing.
A.9.4.2.2
The surface resistivity of a grain conveyor belt affects
its ability to accumulate a charge. Tests have shown that belts
with a resistance of 10
6
ohms to 10
8
ohms are conductive enough
to prevent significant accumulation of charge. According to the
draft standard Safety of Machinery — Electrotechnical Aspects, pub-
lished by the European Committee for Electrotechnical Stan-
dardization (CENELEC), a belt is considered to be sufficiently
conductive if the surface resistance is less than 3 
× 10
8
ohms.
A.9.4.3
According to the draft standard Safety of Machinery —
Electrotechnical Aspects, published by the European Committee
for Electrotechnical Standardization (CENELEC), V belts and
similar transmission belts are considered to be sufficiently con-
ductive if the following criterion is met:
where:
R = 
resistance measured at the inner side of the 
mounted transmission belt between an electrode 
halfway between the two pulleys and ground
B =
width of a flat belt or twice the depth of the side 
face of a V belt
For belts constructed of different materials, the belt is con-
sidered to be sufficiently conductive if the resistance across the
belt does not exceed 10
9
ohms, measured at 23
°C and 50 per-
cent relative humidity.