/images/PowerCableUpdate.jpg/images/southwire_100x70.jpg600V Toolkit: Thick Insulation Handles High Resistance Faults600VTookKitThickInsulationHandlesHiResistanceFaults.htmIn 600V systems with high-resistance grounds, adding insulation thickness can buy you equipment uptime when you need it most.
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600V Toolkit: Thick Insulation Handles High Resistance Faults

Industrial power systems running at 480V may seem pretty straightforward, but there are tricks to every trade. In high-resistance grounded systems, the right selection of insulation thickness can pay off in more predictable plant production. That goes straight to the bottom line and makes plant managers happy.

"Some industrial power systems add a resistor between the system neutral and ground to limit the maximum fault current to ground," says Dave Cooper, Southwire applications engineer. "While reducing the magnitude of overvoltages and eliminating arcing, a major goal of high resistance grounding is to avoid immediate service interruption on the first ground fault. This allows machinery to be shut down systematically, or remain in service for a period of time if the fault is minor."

As an extension of that principal, some engineers select a 2kV cable where a nominal 600V cable could be used. The extra insulation thickness handles voltage anomalies during fault conditions for longer periods of time.

ICEA defines fault-tolerance times

The engineering principle for determining fault-tolerance time is drawn from medium-voltage system design. In medium-voltage systems, engineers can opt for 133 percent or 173 percent insulation thickness to increase fault-tolerance time. The concept is clearly spelled out in ICEA (Insulated Cable Engineers Association) standard S-95-658 for medium-voltage cable.

 In this standard, Paragraph 3.2 deals with insulation levels. 133 Percent Level can be used where "there is adequate assurance that the faulted section will be de-energized in a time not exceeding 1 hour." With even more insulation, you get more time. The 173 Percent Level "should be applied on systems where the time required to de-energize a grounded section is indefinite."

"These insulation levels are chosen to handle increased voltage levels that may be seen in systems when faults occur," Cooper says. "They don't protect against possible heat generated by increased current in the conductor."

The NEC doesn't recognize this medium-voltage engineering option for 600V applications, but many plant engineers apply the same reasoning to the lower-voltage systems as a matter of practical reality.

2kV cable handles 600V fault voltages

Here's an example: In a fault situation, the voltage on one phase of your 600V cable application may increase. If you want to tolerate that fault for a long period of time, consider using a 2kV cable. You may be able to tolerate the fault indefinitely until you can shut down the equipment for maintenance.

Table 3-4 in ICEA Standard S-95-658 supports this thinking. If you review this table and its footnotes, you'll see that for a 500 kcmil conductor running at 600V, 65 mils of insulation is adequate for a 133 percent insulation level, which allows up to an hour of fault operation.

If you need even more fault tolerance time, switching to a 2kV cable provides you with 90 mils of insulation. That's good for an hour of fault operation in a 2kV system, and an almost indefinite time in a 600V system.

"If your plant is in the middle of a critical production run, it may be a week before you can shut the equipment down for maintenance," says Cooper. "If you're running 2kV cable in the system, you can handle those fault voltages for that long. And your plant manager will love you."