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Here is an excerpt from some Schneider Electric installation instructions for their QO2175SB 240V single phase "SurgeBreaker" SPD:

NOTE: The SPD ... must be installed as close to neutral assembly and main circuit breaker or main lugs as possible. Keep wire lengths as short as possible with no sharp bends.

(source)

What is the reason for avoiding sharp bends? Is this more likely to cause the wire to fail under surge conditions? Or would it somehow hurt the surge protection effectiveness?

And how small of a radius bend would be considered "sharp"?

StayOnTarget
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  • Not all copper wire is the same amalgam. Some less scrupulous suppliers use inferior copper mixtures. Regardless of the quality, though, any wire bent sharply can cause fatigue and/or damage either the conductor or the insulation or both. It's just a bad idea to use sharp bends, generally. I think they are just making the point stand out in clear language so that the result is far less likely to exhibit failure. Just follow the instructions and the results will be more professional and reliable. The radius depends on the wire diameter, the copper and insulation construction, etc. – jonk Jan 01 '21 at 20:20
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    If you are in a situation where you are unsure about it, you can try out the bend on a scrap piece of wire and then carefully examine the insulation with a magnifying glass to see if you see damage to the insulation. Then remove the insulation carefully and perform a similar magnified examination of the copper wire. If you see any crazing or discoloration of any kind at all in the copper itself, then it's too tight of a bend. It should look even and pristine. – jonk Jan 01 '21 at 20:22
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    The IPC standard for single wire bend radius is 2x the outer diameter of the wire, which includes any insulation. For the 12AWG THHN I have sitting next to me, for example, that would be a 1/4" radius which seems fairly gentle. I think their biggest concern is the "oh I've got 4" of extra length and I don't want to cut and restrip so I'll squish it into this empty area over here". – vir Jan 01 '21 at 23:57

1 Answers1

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Surges often approximate an "infinite impulse" and, as such, contain significant high-frequency components. In order to be effective you want the surge protection device to shunt as much of that impulse as possible to ground so that it does what you installed it for, specifically to protect your equipment.

A bend of any kind in a transmission line affects its impedance and a sharp bend affects it significantly. You might not think you are employing a t-line but considering the HF components of the surge, it is indeed one.

By keeping it as straight and short as possible you keep the impedance as constant as possible and therefore you reduce reflections which reduce the effectiveness of the surge protection device.

jwh20
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