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Using High-Speed Grounding Switches
Using High-Speed Grounding Switches

Protection of power transformers //

Automatic high-speed grounding switches are applied for protection of power transformers when the cost of supplying other protective equipment is deemed unjustifiable and the amount of system disturbance that the high-speed grounding switch creates is judged acceptable.

The switches are generally actuated by discharging a spring mechanism to provide the ‘‘high-speed’’ operation.

The grounding switch operates to provide a deliberate ground fault on one phase of the high-voltage bus supplying the power transformer, disrupting the normally balanced 120° phase shifted three-phase system by effectively removing one phase and causing the other two phases to become 180º phase shifted relative to each other.

This system imbalance is remotely detected by protective relaying equipment that operates the transmission line breakers at the remote end of the line supplying the power transformer, tripping the circuit open to clear the fault.

This scheme also imposes a voltage interruption to all other loads connected between the remote circuit breakers and the power transformer as well as a transient spike to the protected power transformer, effectively shortening the transformer’s useful life.

Cleaveland/Price's High speed grounding switch
Cleaveland/Price’s High speed grounding switch 115 kV – 230 kV, 120 kA momentary / 71kA, 3 sec. (

Frequently, a system utilizing a high-speed ground switch also includes the use of a motor operated disconnect switch and a relay system to sense bus voltage.

The relay system’s logic allows operation of the motor operated disconnect switch when there is no voltage on the transmission line to provide automatic isolation of the faulted power transformer and to allow reclosing operations of the remote breakers to restore service to the transmission line and to all other loads fed by this line.

The grounding switch scheme is dependent on the ability of the source transmission line relay protection scheme to recognize and clear the fault by opening the remote circuit breaker.

Clearing times are necessarily longer since the fault levels are not normally within the levels appropriate for an instantaneous trip response.

500 kV Motor Operated Disconnect Switch (VIDEO)

Can’t see this video? Click here to watch it on Youtube.

The lengthening of the trip time also imposes additional stress on the equipment being protected and should be considered when selecting this method for power transformer protection.

High-speed grounding switches are usually considered when relative fault levels are low so that the risk of significant damage to the power transformer due to the extended trip times is mitigated.

Resource: Electric Power Substations Engineering – J. D. McDonald (Get it from Amazon)

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Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


  1. Harold Eddins
    Aug 15, 2019

    One of our utilities customer has several explosive propellant cartridges for use with Siemens Allis 640 KV High Speed Grounding Switch HSG. We are their hazardous waste disposal contractor and are gathering information on these cartridges so they can be properly disposed of. The cartridges are manufactured by Propellex Corp – Edwardsville, IL and these were made in 1990. We assume the cartridges are used as actuators in the switches. Our research has come up empty thus far. Can you shed some light on these cartridges, either general or specific, that can help us? I can send what info. I have and pictures. Thanks.

    • Eric Gruber
      Apr 24, 2020

      I’m looking to procure these Siemens Allis ground charges. I have some 230kv HSG switches in my system that could possibly use the charges you are trying to dispose of.
      if you still have them, do you have any pics that youcan send me.

  2. Dhruv Patel
    Oct 16, 2017

    Hi, I have a visio file that shows the circuit you are referring to in this article. It would be nice to show a single line diagram of the remote CB, tx line, distribution substation, isolation switch, HSGS, etc. Can I contribute to this article by offering you a visual? I don’t care about my name being displayed on the article.

    • Edvard
      Oct 16, 2017

      Hi Patel, thanks for taking your time to comment. Sure you can send me SLDs or pics related to this article and I will update technical article. And the name… don’t worry about that, every contributor is named :)

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