Arcing - What affects size of arc when switch is opened?

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Discussion Overview

The discussion centers around the phenomenon of arcing that occurs when an electrical circuit is opened or closed, specifically investigating the factors that influence the size of the arc, such as the presence of resistance or load in the circuit. The scope includes theoretical considerations and practical implications related to electrical engineering and circuit design.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions whether the size of the arc is greater when there is a resistance/load on the circuit, suggesting that it likely is, but seeks clarification on the effects and calculations involved.
  • Another participant notes that the entire circuit must be considered, indicating that inductances, high switching voltages, and large currents generally increase arcing.
  • A different participant asserts that without a resistance/load, there would be no circuit and thus no arc at all, challenging the initial premise.
  • There is a discussion about whether a main breaker closing would cause arcing if the individual circuits are open, with conflicting views on whether voltage alone can cause arcing without current.
  • One participant confirms that if the individual circuits are open, there would be no arc when the main breaker is closed, while another participant disagrees, stating that it will arc despite the absence of current.
  • A later reply introduces the concept of "no load disconnect" and "load disconnect" switches, explaining their operational interlocks and the specific design considerations for medium voltage switches, including the presence of arcing contacts and arc chutes.

Areas of Agreement / Disagreement

Participants express differing views on whether arcing occurs without current, leading to unresolved questions about the conditions under which arcing happens. There is no consensus on the effects of load on arcing size, and multiple competing perspectives remain throughout the discussion.

Contextual Notes

The discussion highlights the complexity of arcing phenomena, including dependencies on circuit configurations, load conditions, and the specific types of switches used. Assumptions about the relationship between voltage, current, and arcing are not fully resolved.

Crazyhorse2882
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When an electrical circuit is opened or closed there is an arc between the terminals/buss that the switch or breaker is connecting or disconnecting and the arc is greater depending on the amount of voltage. My question is, is the arc greater when there is a resistance/load on the circuit? I'm guessing yes, but what is the effect and how could you calculate it?
 
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You have to consider the whole circuit to determine that.
In general, inductances (especially coils), large switching voltages and large currents will increase the amount of arcing you get.
 
Crazyhorse2882 said:
... is the arc greater when there is a resistance/load on the circuit? ...

Without a resistance/load, you wouldn't have a circuit in the first place, and therefore wouldn't have an arc at all.
 
So then if a main breaker was closed outside of a building feeding equipment inside the building but their individual circuits were open then there would be no arc as that breaker closed? There would be voltage but no current and my understanding is that it will arc
 
Crazyhorse2882 said:
So then if a main breaker was closed outside of a building feeding equipment inside the building but their individual circuits were open then there would be no arc as that breaker closed?
That is correct.
There would be voltage but no current and my understanding is that it will arc
No, it will not arc.
 
To emphasize this point - for medium voltage switches - there are two general classes "no load disconnect" and "load disconnect" - The no load types are ideally interlocked ( often literally with locks and keys - ref Kirk Key) - with the load side circuit breaker. The Load side circuit breaker must be open - to get the key that allows the line side "no load" switch to be operated ( opened or closed).
TO feed a transformer a MV switch used on the primary is technically a load break switch (it will have an arc chute & arcing contacts- separate form the main contacts) , but only rated to interrupt the magnetizing current of the transformer it feeds - so you still see the interlocking scheme.
 

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