IE13: Static electricity initiated by high voltage

Hazards: In electrostatic application processes, the electrically generated electrostatic charge can result in spark-over with ignition of an explosive atmosphere if present.

Protective measures: Limit the energy of the spray system to < 0.24 mJ or design-based explosion protection (chapter CS)

During electrostatic liquid coating application processes, the liquid coating material is converted into a mist of paint droplets and deposited on the workpiece with the aid of the forces in an electric field. To this end, the droplets are charged by means of a high voltage of the order of several tens of kilovolts, with the result that they are attracted by the earthed workpiece. The coating material can be sprayed by means of either compressed air, hydrostatic pressure or a combination of these processes and other centrifugal forces.

The static electricity initiated by high voltage is an operational ignition source as a function of the performance specifications of the spray systems for the application process:

EN 50171 specifies a distinction between the following types:

ESTA spray system	Energy restriction	Current restriction	Risk of personal injury	Ignition capability	Design-based protective measures
Type A	< 0.24 mJ	N/a	No	No	N/a
Type B	0.24 to 350 mJ	0.7 mA	No	Yes	Physical protection, see chapter CS
Type C	> 350 mJ	> 0.7 mA	Yes	Yes	Physical protection, see chapter CS

In the context of electrostatic (ESTA) application, type B and C spray systems must always be regarded as operational ignition sources.

Correspondingly, this ignition source is inadmissible without supplementary measures for a prevailing Zone 2 onwards.

Since the presence of this ignition source is a prerequisite for the electrostatic coating process as a result of the process design, the resulting hazard cannot be combated with preventive explosion protection measures.

ESTA systems with type B and C spray equipment must therefore always be fitted with design-based protection in the form of a physical protection system (see chapter CS). Furthermore, spray equipment of type C must be designed such that walk-in access is not possible. It is essential to ensure the following conditions before switching on the high voltage (only by means of a key) as a detailed concept for complying with the requirements of EN 50176:

• Physical protection system (CS) is functional.
• Ventilation unit is operational, e.g. monitored to ensure:
  • 5-times air change takes place (purging quantity/time monitor)
  • minimum exhaust air volume is attained (air volume monitor)
• The safety zone around the unit is free of personnel (door monitor)  

All metallic workpieces, provided they are not parts carrying a high voltage operationally, must be metallically connected to each other and connected to the power supply’s earth system via the high voltage supply’s earth terminal. The maximum admissible earth leakage resistance may not exceed R = 1MΩ (see chapter IE1).

The components carrying a high voltage must have discharged to a discharge energy of less than 350 mJ within 2 seconds of switching off the high voltage.

The following requirements as specified in EN 50053 apply to ESTA application systems in general with reference to equipotential bonding:

Metallic workpieces (not live)	Earthing R ≤ 1MΩ
Metallic components carrying high voltage operationally	After switching off high voltage, discharge energy < 350 mJ in 2 seconds
Conductive paint feed vessels connected to high-voltage supply in normal operations	Installation in enclosed electrical workshop or protective measures to avoid contact with paint feed vessel
Other conductive components for paint feed system	Either: earthing R ≤ 1MΩ or: connection with high-voltage supply
Non-conductive paint feed vessel	Metallic contact between coating material and conductive component, with dissipation of charge to discharge energy < 350 mJ within 2 seconds of switching off high voltage
Non-conductive paint feed lines, carrying high voltage in normal operation	Breakdown test on sample of line 2.5 m long with 1.5x maximum voltage

Example: Combined automatic application unit with electrostatic and pneumatic application system