Static electricity shall be controlled or eliminated in areas where materials are processed or handled that are ignitable by static spark discharge. This category includes spark-sensitive explosives, propellants, and pyrotechnics, as well as solvent vapors and flammable gases. Approved systems to dissipate static electricity shall conform to the requirements of NFPA 77 and IEEE 142.

5.1.4.1 BONDING AND GROUNDING EQUIPMENT

Bonding straps shall be used to bridge locations where electrical continuity may be broken by the presence of oil on bearings, or by paint or rust at any contact point. Permanent equipment in contact with conductive floors or tabletops is not considered adequately grounded. Static grounds shall not be made to gas, steam, or air lines; dry-pipe sprinkler systems; or air terminals of lightning protection systems. Any ground that is adequate for power circuits or lightning protection is more than adequate for protection against static electricity.

5.1.4.2 TESTING EQUIPMENT GROUNDING SYSTEMS

Grounding systems shall be tested for electrical resistance and continuity when installation is complete and, in the case of active equipment, at intervals to be locally determined. The grounding system shall be visually inspected for continuity (before reactivation of the system) if the equipment has been inactive for more than 1 month. All exposed explosives or hazardous materials shall be removed before testing. During a test for resistance to ground, all equipment, except belt-driven machines, shall be considered as a unit. In measuring the total resistance to ground for belt-driven machinery (to ensure compliance with Section 5.1.4.3), resistance of the belt is to be excluded. All conductive parts of equipment shall be grounded so that resistance does not exceed 25 ohms, unless resistance is not to exceed 10 ohms because of the lightning protection system. For existing equipment, the rate of static electricity generation shall be considered before changes are made in grounding systems. The resistance of conductive rubber hose shall not exceed 250,000 ohms. (See NFPA 77 and NFPA 780 for further information.)

5.1.4.3 CONDUCTIVE FLOORS, SHOES, MATS, AND WRISTBANDS

Conductive floors and shoes should be used for grounding personnel conducting operations involving explosives that have an electrostatic sensitivity of 0.1 joule or less. Many flammable liquids and air mixtures can be ignited by static discharge from a person. In areas where personnel come close enough to have possible contact with static-sensitive explosives or vapors, conductive floors shall be installed except where the hazards of dust-air or flammable vapor-air mixtures are eliminated by adequate housekeeping, dust collection, ventilation, or solvent-recovery methods. Conductive floors may also be required where operations are performed involving EEDs that contain a static-sensitive explosive.

Conductive floors are not required throughout a building or room if the hazard remains localized. In such cases, conductive mats or runners may suffice. These mats or runners shall be subject to all the specifications and test requirements that apply to conductive floors. Conductive wristbands may be substituted for conductive mats and footwear at fixed, grounded workstations or outdoor location.

5.1.4.4 SPECIFICATIONS FOR CONDUCTIVE FLOORS AND WRISTBANDS

Conductive floors shall be made of nonsparking materials such as conductive rubber or conductive flooring material and shall meet the following requirements:

1. The flooring and its grounding system shall provide for electrical resistance not to exceed 1,000,000 ohms (measured as specified in Section 5.1.4.5).

2. The surface of the installed floor shall be reasonably smooth and free from cracks. The material shall not slough off, wrinkle, or buckle under operating conditions. Conductive tiles are not recommended for use in areas where contamination can be caused by explosive dust. The large number of joints and the tendency of tiles to loosen provide areas where explosive dust can become lodged and that are not easy to clean with normal cleaning procedures.

3. Where conductive floors and shoes are required, resistance between the ground and the wearer shall not exceed 1,000,000 ohms, which is the total resistance of conductive shoes on a person plus the resistance of floor to ground. Where conductive floors and shoes are required, tabletops on which exposed explosives or dust are encountered shall be covered with a properly grounded conductive material meeting the same requirements as those for flooring.

4. Conductive floors shall be compatible with the explosive materials to be processed.

5. Conductive wristbands shall not exceed a resistance of 1,000,000 ohms between the wearer and ground. This resistance shall be measured with a suitably calibrated ohmmeter. Wristbands shall be of a design that maintains electrical contact with the wearer when tension is applied to the ground lead wire or the wristband is placed under strain.

5.1.4.5 CONDUCTIVE FLOOR TEST

Before use, tests shall be conducted on all conductive floors; subsequent tests shall be made at least semiannually. Test results shall be permanently recorded and a copy filed in a central location. Instruments used in testing shall be used only when the room is free from exposed explosives and mixtures of flammable gases.

Maximum floor resistance shall be measured with a suitably calibrated insulation resistance tester that operates on a normal open-circuit output voltage of 500 V do and a short-circuit current of 2.5 mA with an effective internal resistance of approximately 200,000 ohms. Minimum floor resistance shall also be measured with a suitably calibrated ohmmeter.

Each electrode shall weigh 2.3 kg and shall have a dry, flat, circular contact area 6-1/2 cm in diameter, which shall comprise a surface of aluminum or tinfoil 1.3 to 2.5 mm thick, backed by a layer of rubber 0.6 to 0.65 cm thick, and measuring between 40 and 60 durometer hardness as determined with a Shore Type A durometer (ASTM D-2240-68).

The floor shall be clean and dry. Only electrode jelly shall be used to establish a good contact. (Brushless shaving soap and saline solution shall not be used.)

The resistance of the floor shall be more than 5,000 ohms in areas with 110-V service, 10,000 ohms in areas with 220-V service, and less than 1,000,000 ohms in all areas, as measured between a permanent ground connection and an electrode placed at any point on the floor and also as measured between two electrodes placed 3 ft apart at any points on the floor. Measurements shall be made at five or more locations in each room. If the resistance changes appreciably during a measurement, the value observed after the voltage has been applied for about 5 sec shall be considered the measured value. (See Figure 5-1.)