I've just learned about the surge protection device (SPD). As I was reading about its connection in a three phase line, I came across this scheme:

How it is

I understand that the SPD's are there to limit the line tension in respect with the PE reference, but why isn't it connected to the line neutral? Each phase's voltage should be measured like this in my mind:

How I see it should be

Also, isn't the protective ground meant to protect things that are outside the circuit? like other electronics, people etc. against accidental contacts?. How can you intentionally connect the circuit with a reference that is only there for safety?


When connected to Earth Ground as opposed to Neutral ground, the intent is to eliminate Neutral additional inductance and suppress transient noise to ground to reduce EMI interference to nearby radios and mobiles.

Otherwise every switch transfer would sound like lightning hundreds of miles away on AM radio.

the Q of the RLC values depends greatly on R at resonant frequency as the SPD is generally very low Zzt.

  • \$\begingroup\$ but we are talking about a three phase line tension (more than 100 V) vs a small radio interference. The SPD is meant to protect agains huge surges of tension, for example a lightning striking nearby. And in my case, I'm studying to implement it on a motor powered by a generator, so the distance to the neutral is not that far. \$\endgroup\$ – Arthur May 24 '17 at 16:28
  • \$\begingroup\$ True but so the shortest path to ground is Earth ground and not Neutral where ESL and EMI are also important. If your case is unique then ok. Yes and we are talking about big dv/dt and di/dt so both E&H field noise are dominant. Industrial noise pollution. I consulted for a transformer factory once and the Xenon strobe safety flash during HV tests caused all mobile radio reception to be blocked (jammed) (EMI pollution) ( bad design) \$\endgroup\$ – Sunnyskyguy EE75 May 24 '17 at 18:17
  • \$\begingroup\$ but beware of earth impedance currents for Vrise and safety. \$\endgroup\$ – Sunnyskyguy EE75 May 24 '17 at 18:24
  • \$\begingroup\$ Ok. So this is the only reason the SPD is not connected to the neutral? \$\endgroup\$ – Arthur May 24 '17 at 21:25
  • \$\begingroup\$ Vrise or GPR is explained in a lot more detail here nemasurge.org/faqs/#grounding \$\endgroup\$ – Sunnyskyguy EE75 May 24 '17 at 21:38

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.