Why Are Surge Capacitors Tied to Ground?

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Discussion Overview

The discussion revolves around the function and grounding of surge capacitors in electrical systems, particularly in relation to their role in managing transient voltages and protecting circuits. Participants explore the theoretical and practical implications of surge capacitors, including their interaction with inductance in motor circuits.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant seeks clarification on the function of surge capacitors, questioning their grounding and whether they provide capacitance to offset inductance in motor circuits.
  • Another participant suggests that surge capacitors absorb energy from voltage spikes and discharge back into the system, noting that this is not a significant concern.
  • A different viewpoint introduces the concept of lightning strikes as a source of DC surges, prompting a question about how to design a grid to handle such events.
  • One participant explains that surge capacitors are selected to provide a low impedance path to ground at high frequencies, which helps divert surge currents away from sensitive circuit components, particularly in the context of external sources like lightning.

Areas of Agreement / Disagreement

Participants express various viewpoints on the function and grounding of surge capacitors, indicating that multiple competing views remain and the discussion is not resolved.

Contextual Notes

There are limitations regarding the assumptions made about the nature of transients and the specific design considerations for surge capacitors, which are not fully explored in the discussion.

kenneth edmiston
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I have a decent understanding of power factor caps, (far from perfect) but I’m not as clear on surge caps.

Surge caps store transient voltages and discharge back into the system?
Why are they tied to ground?
And do they also provide capacitance in the motor circuit they are tied to providing an offset to inductance?

Thanks
 
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Generally - they just absorb the energy of a voltage spike, and technically discharge back to the system, but this is not a significant issue.

Since the V spikes - could be on a single phase, you want this protection relative to ground.
 
Think lightning strikes. They are DC. If you designed the grid, how would you deal with lightning surges?
 
The transients in power systems that surge capacitors are intended to eliminate are composed of higher frequencies than the 50/60/400 Hz power frequency. So the surge capacitor is selected to provide a low impedance path to ground at high frequencies to divert the surge currents away from the part of the circuit you want to protect. Most higher frequency transients coupled into the power system can be diverted to ground (earth) because they come from external sources, like lightning, for example.
 

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