January 1, 2005
From engineering studies, preliminary
engineering, and requirements gathered through commissioning and ongoing maintenance, system integrators and end users have more than enough on their plates to ensure the successful implementation of a control and information system project.
But, still, there are issues that remain
“outside” a project’s traditional lifecycle continuum that you cannot ignore.
Not the least of which are areas in a plant considered by the National Electrical Code as hazardous, either from flammable vapors that might be present (Class I), or from fine-grain dust in the air (Class II). In these hazardous areas, equipment grounding becomes even more important than ever.
What is the most dreaded thing in these hazardous areas? An arc or a spark.
How does a poor equipment grounding system contribute to this?
First of all, if there is a fault in the
electrical system, there is likely to be an arc. If the insulation on a conductor
wears through or a motor fails, the arc
should remain inside the conduit system
initially.
What is wanted is the circuit to clear as quickly as possible. How quickly depends largely on just how good the equipment grounding system is. A few ohms, or perhaps only a few tenths of ohms, introduced by a loose connection, a rusted coupling, a poor flex connection, etc., could stretch out the operating time of the circuit breaker. These few seconds of extra time could allow the arc of the fault to burn through the conduit system or the motor housing, and then expose the arc to the hazardous atmosphere.
The result is a big fire and another fire marshal's report that determines the cause to be "faulty wiring."
In these hazardous areas, there are
special, more stringent rules regarding
equipment grounding. If the motor is in
the hazardous area, liquid-tight flexible
metal conduit cannot be relied on to make the required ground connection. It must bond with a properly terminated
conductor – a "jumper bond” – either inside or outside of the flex. Also,
you cannot rely upon standard knockouts,
locknuts, or bushings for good connections
all the way back to where the equipment
ground bus ties to the neutral bus, usually at the main switchboard. All these intervening connections must bond with a proper-sized conductor and proper terminations.
As stated, a user must size the jumper
bond properly. In each case, there is a
circuit breaker somewhere in a properly
designed circuit that should operate if
there is a fault. As described earlier, the fault path resistance will determine how much fault current will flow, and the
circuit breaker time-current curve will
determine how much time it will take for the circuit to clear. The bonding jumper must carry this current for that length of time without melting, or it still won't clear the fault.
Most circuits utilize alternating current
(AC). Without going into a detailed
engineering description of what's involved, it is advisable to proceed with
the understanding that wires conducting
AC current have magnetic properties that have to go into consideration during design and installation.
The general rule that affects grounding is the resistance to current flow will
increase if all the circuit conductors are
not near each other. During a fault, that also includes the equipment grounding conductor. That is why we can’t just tie equipment to the nearest steel beam or drive a ground rod next to each light pole to consider them grounded.
Some rules to keep in mind to help ensure a fire-safe control system environment:
Routing of grounding conductors is very important to assure the proper operation of the circuit protective devices.
Hazardous areas require special attention
to the National Electrical Code rules for the grounding to minimize the potential for explosion and fire.
Darrel Ramhorst is Chief Operating
Officer of Interstates Control Systems
