Download Grounding The Powder Coating System

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TECHNICAL BRIEF
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THE POWDER COATING INSTITUTE
2121 EISENHOWER AVENUE, SUITE 401,ALEXANDRIA, VIRGINIA 22314
GROUNDING THE POWDER COATING SYSTEM
Proper grounding of a powder coating system is absolutely essential for safe operation and quality
finishing. Powder coating particlesare electrostaticallycharged by passing through an electrostatic
field or by frictional charging (tribo charge). The charged particles are attracted to a grounded
surface (the part to be coated). Without a properly grounded surface available, a majority of the
powder particles will not adhere to their target and will be drawn into the powder recovery system.
PRODUCTION PROBLEMS CAUSED BY POOR GROUNDING
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Inconsistent coating. A situation where one or more parts, in a series of parts, is not
coated as evenly as the others. Faraday area problems may also be more pronounced when the
part grounding is poor. If grounding is poor throughout the coating area then all the parts will be
inadequately coated.
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Poor transfer efficiency. Improperly grounded parts will not attract powder as they
should. Less powder will adhere to the part, lowering first pass transfer efficiency.
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Inadequate film thickness. Apart with insufficient grounding may hold the same charge
as the powder particles and reject (back ionize) additional powder particles sprayed on the surface.
This slows, or eliminatesthe build-up of powder needed to attain the proper coating thickness. This
may be especially important when a coating of more than 2 mils is required.
SAFETY PROBLEMS CAUSED BY POOR GROUNDING
All equipment in the powder coating operation and area (as defined by The National Fire Protection
Association, NFPA Bulletin Number 33, Chapter 13) must be grounded for safe operation. Any
ungrounded object will store an electrostatic charge when it is subjected to an electrostatic field.
When this charge reaches a sufficient level the energy will be discharged to ground causing an arc
or a spark. This may be a source of ignition that, in the presence of the right fuel to air mixture,
has the potential to cause a fire. If this occurs in a confined space the result can be an explosion.
Equipment that must be grounded includes, but is not limited to:
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the parts being coated
the spray booth
the recovery equipment
the spray equipment
the gun motion equipment
ductwork (if used)
feed hopperlsystems
the conveyor and part hooks
the operator or manual spray personnel
equipment and/or operator platforms
vacuum cleaning systems
The material provided in PCI Technical Briefs i s for general informational purposes only.
Issued 4/92 Always consult an expert in powder coating before attempting actual applications.
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A PROPER GROUND
NFPA Bulletin Number 33, Chapter 13, paragraph 13-4c, states, 'To minimize the possibility of
ignition by static electrical sparks, powder transportation, application, recovery equipment, work
pieces and all other conductive objects shall be grounded with a resistance to ground not
exceeding one megohm." (One megohm is 1,000,000 ohms.) This is the definition of a proper
ground.
Uncoated metal parts, clean hangers and clean conveyors have very little resistanceto ground and
are excellent electrical conductors. What can degrade this ideal path to ground is powder buildup on the contact points of the hangers or conveyor. Also the conveyor may have swivels or
indexing assemblies that, when contaminated, can also impede grounding. All of these sources
must not exceed one megohm resistance to ground.
MEASURING PROPER GROUND
The device used to measure continuity to ground is an ohm meter that has a megohm scale. This
can be a volt/ohm meter (VOM) or a Megger. A VOM is adequate for checking electrical circuits,
but its low voltage power source makes it less suited for checking the proper grounding of a
powder coating system. The best device is the Megger which has a power source of 500volts or
higher. This higher voltage provides the current required to accurately measure the resistance to
ground.
The proper technique for measuring resistance is to start at the end of the process and work
backward. The meter is connected between a known building ground and the uncoated part to
be tested using a long test lead. This procedure is used to determine that the part is correctly
grounded through the entire spray booth. The amount of resistanceto ground can be read on the
meter.
Becausethe meter is attached to a known ground and to a clean part on the conveyor in the booth,
all of the devices in between....hanger, conveyor, swivels etc., are in the circuit and the resistance
to proper ground can be measured. If the reading is less than one megohm, the grounding is
correct.
If the resistance reading is greater than one megohm, hook the lead to the contact point on the
hanger and read again. Repeat this procedure, working back through the system (swivel or
conveyor hook, conveyor) until the resistance reads in the proper range. By this method you can
determine which device needs corrective action.
A similar technique should be used to check for proper grounding of other objects and equipment
in the powder coating area.
IMPROVING THE GROUND
Clean, uncoated metal parts should present no grounding problems. The place to begin looking
for excessive resistanceto ground is the hook or hanger that carries the parton the conveyor. The
key parts of the hanger are the contact points (both part and conveyor contact points).
To assure good grounding, routine hanger maintenance is very important. Maintenance can be
performed in various ways:
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clean contact points only
burn-off the entire hanger
chemical strip the entire hanger
sandblast the entire hanger
mechanically break the coating off the hanger
If the conveyor itself appears to be a problem it may be necessary to provide a new ground in the
area of the spray booth. This will require that the conveyor rail be wired directly to a known building
ground. If after this is done a problem still exists, check the conveyor wheels, chains, swivels, or
indexing devices for cleanliness. If they need cleaning or replacement consult the conveyor
manufacturer for the proper procedure.
If all else fails.... a grounding bar may be required. This may be a bar or a brush connected to a
known building ground that conducts a proper ground direct to the part hanger. This device brings
with it some potential problems which include the possibility of metal shavings in the spray booth,
impedimentsto color change, noise generationand the creation of a trap for dirt and contaminants.
The importance of proper grounding of the powder coating system cannot be overstated. Poor
grounding will have an impact on the entire powder coating operation affecting quality, efficiency
and safety.
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