non-metallic box with metal connector

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mrd999

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I have a panel surge protector - a sealed non-metallic nema 4x enclosure with a dual threaded nipple at the exit point of 4 wires. The manufacturer indicates it may be installed directly to a panel knockout or via conduit. The install will be via conduit due to orientation.

I find no continuity between the ground wire and the connector threads, so I assume I must use metal conduit, rather than nonmetallic, from the panel to ground the connector. I'd prefer the use of nonmetallic, but without having a way to remove the threaded fitting, I don't see an alternative.
 
I would assume the threads need to be grounded so that is why that want direct connection to a panel or via conduit. I suspect that the conduit can't be very long either.
 
jumper said:
Why?
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The surge protective device (SPD), itself is not what I'm talking about. My statement is only about the leads.

The faster the transient, the greater the reactive impedance of the leads. DC resistance of the leads is an almost inconsequential factor.

I don't have formulas or a reference at hand that I can offer, but I can attest to witnessing some great math in a power point presentation. But I'll bet someone here can lay it out.

The rise time of the current waveform is the primary thing to pay attention to. The steeper the slope of the current waveform, the greater the SPD lead reactive impedance effects.
 
jumper said:
Why? I am curious. I know very little about surge protection devices.
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Look at 285.12. In a nut shell you want the conductors as short and as streight as possible. With a surge you do not want resistance from conductor length or bends. You want the surge to get to the devise as quick as possible.

mrd999 said:
I have a panel surge protector - a sealed non-metallic nema 4x enclosure with a dual threaded nipple at the exit point of 4 wires. The manufacturer indicates it may be installed directly to a panel knockout or via conduit. The install will be via conduit due to orientation.

I find no continuity between the ground wire and the connector threads, so I assume I must use metal conduit, rather than nonmetallic, from the panel to ground the connector. I'd prefer the use of nonmetallic, but without having a way to remove the threaded fitting, I don't see an alternative.
Click to expand...

250.110 states exposed NCC metal parts likely to be energized shall be connected to the EGC. I dont think metal threads molded into the box would meet the condition.
 
al hildenbrand said:
The shorter the leads, as a rule, the better the performance of the surge protective device.
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Also,

Ditek Corp TVSS/SPD specs have the ground lead off the unit the shortest.

I guess that reduces the circuit length, even if only a few inches, which reduces resistance.
 
nhfire77 said:
I guess that reduces the circuit length, even if only a few inches, which reduces resistance.
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It's not about resistance, it's about impedance.
Surge transients have a high frequency component, 1MHz or higher.
Wire has a self-inductance and the longer the wire the greater the self-inductance.
So the high frequency part of the transient sees this self-inductance as a high impedance.
 
Speedskater said:
It's not about resistance, it's about impedance.
Surge transients have a high frequency component, 1MHz or higher.
Wire has a self-inductance and the longer the wire the greater the self-inductance.
So the high frequency part of the transient sees this self-inductance as a high impedance.
Click to expand...

And loops or sharp bends in the wires add more inductance.
 
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