Wednesday, August 20, 2008

Using Technology to Reduce Risk and Improve Worker Safety-Part 2

The choice for many engineers is focussed on what grounding technology to use.
A solidly grounded system is one in which one conductor of the system has been intentionally connected to earth ground with a conductor having no intentional impedance and this partially reduces the problem of transient overvoltages found on the ungrounded system.
While solidly grounded systems are an improvement over ungrounded systems, and speed the location of faults, they lack the current-limiting ability of resistance grounding and the extra protection this provides. The destructive nature of arcing ground faults in solidly grounded systems is well known and documented and are caused by the energy dissipated in the fault. A measure of this energy can be obtained from the estimate of kilowatt-cycles dissipated in the arc:
Kilowatt cycles = V x I x Time/1000.
In the same IEEE Standard as referenced above, section 7.2.2 states that: "one disadvantage of the solidly grounded 480-V system involves the high magnitude of ground-fault currents that can occur, and the destructive nature of arcing ground faults."
Since the vast majority of arcing faults start their life as single-phase faults, the key to reducing their impact is to use technology that either significantly reduces the fault current level thereby reducing the magnitude of the arc hazard and/or using technology that prevents transient overvoltages that can lead to single-phase faults escalating into arcing faults.
The answer in both cases is high resistance grounding, as recognized in the Canadian Electrical Code section 10-1100, and the National Electrical Code section 250.36.
High resistance grounding of the neutral limits the ground-fault current to a very low level (typically from 1 to 10 amps) and this is achieved by connecting a current-limiting resistor between the neutral of the transformer secondary and the earth ground and is used on low-voltage systems of 600 volts or less, under 3000 amp. By limiting the ground-fault current, the fault can be tolerated on the system until it can be located, and then isolated or removed at a convenient time.
In tests, damaging voltage transients measured on a 480-volt ungrounded system were eliminated once the circuit was converted to high resistance grounded.
With respect to the magnitude of fault current, the energy or I 2 t value for a 1 amp fault is 1/ 1,000,000 of a 1000 amp fault assuming an equal amount of time.
The National Electric Code allows a fault level of 1200 amps for one second on a solidly grounded system before a circuit must trip; however, in practice fault levels in excess of 20,000 amps are common for a short period of time.
In IEEE Standard 142-1991 Recommended Practice for Grounding of Industrial and Commercial Power Systems the following perspective is outlined in section 1.4.3:
"The reasons for limiting the current by resistance grounding may be one or more of the following.
1) To reduce burning and melting effects in faulted electric equipment, such as switchgear, transformers, cables and rotating machines.
2) To reduce mechanical stresses in circuits and apparatus-carrying fault currents.
3) To reduce electric-shock hazards to personnel caused by stray ground-fault currents in the ground return path.
4) To reduce the arc-blast hazard to personnel who may have accidentally caused or who happen to be in close proximity to the ground fault.
5) To reduce the momentary line-voltage dip occasioned by the occurrence and clearing of a ground fault.
6) To secure control of transient overvoltages while at the same time avoiding the shutdown of a faulty circuit on the occurrence of the first ground fault."
The judicious use of high resistance grounding facilitates process continuity, reduces equipment damage, allows for predictive maintenance, reduces shock hazard and can minimize the impact of arc blast hazards.

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