I saw wire conduit being installed in a building, and it was metallic. Wouldn't you want to use an insulating material instead? What if the live wire sheath started to fail? It would short to the metal conduit. Isn't that something worth avoiding?
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3 Answers
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The conduit will be connected to earth. If the insulation breaks down, the live cable will short to earth, drawing a large current that will trip a breaker.
If the conduit was an insulator the fault would go unnoticed.
The cable in the conduit may well be a steel reinforced cable that will have the conductors surrounded earthed steel. If the live insulation did break down it would would then short to the steel and again trip a breaker, so in this case the insulated conduit would be unnecessary too.
answered Jun 20, 2019 at 1:21
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It can be metal or plastic as offered in your local hardware store.
I believe all exposed wiring in commercial settings are required to be in conduit according to code. There is no specification of type of conduit I believe so there is no requirement for it to be metal at all.
I believe contractors would prefer metal due to it's ease of use because it can be bent to fit around corners, etc where as plastic requires fittings for every bend. Also I think plastic is used for more weather resistant applications vs metal.
Metal conduit is always grounded to prevent it from being live and will usually trip a breaker if an exposed live wire were to touch it. (One reason why wiring is always sized for the breaker or higher loads)
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Building entrance cables may reduce radiated noise from say arc welders and also improve shunting stray lightning arcs to earth ground by this means of earth ground shielding or sheath in the conduit.
Outside and underground Distribution wires may be coaxial shielded XLPE cables with an outer layer of insulation to impede lightning current induced.
Fault protection gear will protect the customer entrance 1st and/or the switchgear and transformers to determine the optimal OCP time constants.
So it is often, something worth having, rather than avoiding.
Buried entrance is better for lightning avoidance but worse for winter thaw moisture creepage breakdown and often causes power failures in my residential region (maybe once a year) every spring on a warm day after frost is leaving the sub-ground. Power is often restored quickly but these are the tradeoffs for buried in moist clay that freezes in Canada vs above ground entrance. Yet we have one of the best power utilities in the world. So these types of interruptions are added to the maintenance tasks of replacing 30y old XLPE underground cables.
