8/11/2019 Cables Para PTAR

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Waste-Water Appl ications Go for Thicker Insulations

In harsh environments, thicker is better when it comes to power cable insulation.

Power cables lead a tough life for in waste-water treatment plants. The chlorine used in the treatment processcreates an acidic environment that can be hard on cables especially PVC-insulated THHN conductors.

The harsh conditions around waste-water processing motivate many treatment facility operators to switch fromTHHN-rated conductors to cables carrying an XHHW or RHH/RHW type conductor, says David Cooper, Southwiresenior product engineer.

Whats the difference? Any of them can be used in cable trays (if listed) or in conduit. All of them are rated forcontinuous 90C operation in wet and dry locations. But there are differences in materials properties and insulationthickness and those differences count in harsh environments.

How do you choose?

Southwire type THHN or THWN-2 conductors carry a polyvinyl chloride (PVC) insulation thats protected by a nylonjacket. The nylon jacket offers good resistance to oil and gasoline. But if you nick the nylon during installation, youget a couple of problems. First, the PVC insulation can suffer from exposure to chlorine and other chemicals

commonly used in waste-water treatment. The exposure to chemicals is especially critical when you pull THHN intoconduit, because any barbs left in the conduit can cut into the nylon jacket and a lot of THHN goes into conduit.

RHH/RHW-2 and XHHW are showing up as alternatives to THHN in acidic environments for two reasons: They bothuse cross-linked polyethylene (XLPE) insulation rather than PVC. Second, they both offer more insulation thicknessthan THHN. XHHW offers a little more thickness, RHH/RHW offers quite a bit more. In general, in the demandingwaste-water treatment environment, the thicker the insulation, the better off you are.

Heres an example: A single-conductor 350 kcmil THHN cable uses 60 mils of PVC. An XHHW conductor delivers 65mils of XLPE. RHH-RHW gives you 95 mils of XLPE. The table below summarizes the constructions.

Thicker is better

The cost differential for the upgraded insulation is very small compared to the cost of unscheduled downtime,

Cooper says. And for maximum performance, Southwire can supply XHHW insulation with a PVC jacket. On a350kcmil conductor, you get 65 mils of XLPE and a 65 mil PVC jacket, for a total of 130 mils of protection. That willabsorb a lot of abuse.

Table: 600V Power Cable Comparison