Why do circuit breakers rely on stored energy to aid opening/closing, in some cases are filled with oil or gas and have internal sacrificial structural features?
Eur Ing Dr Robert Brown of Fraser George and Associates Limited, is presently involved with a case where a high voltage circuit breaker has failed in the process of closing electrical contacts to re-energise a section of the electrical distribution circuit of a large manufacturing facility. The failure triggered a significant explosion and damage to the breaker and substation.
When considering the aftermath of the ‘explosion’ to the fabric of the substation, the damage incurred by the breaker and the electrical apparatus associated with and connected to the breaker, it was/is apparent that the manifestation and proliferation of ‘electrical arcing’ was fundamentally the cause of the devastation, i.e. the fabric of the substation became structurally unsound due the expanding pressure wave generated and emanating from the point of arcing and significant parts of the breaker and bus bar system to which the breaker was connected had been eroded by the high temperature arc plasma.
The manifestation of electrical arcing incurs other, devastating secondary circumstances such as,
Fire ignition, given suitable combustible material in the vicinity of arc ignition.
Arc flash and burning to individuals close to the location of arc ignition due exposure to hot gasses and fragments of molten metallic materials inherent in the expulsed and expanding arc plasma from the root of arc formation, i.e. arc electrodes.
Erosion of the electrodes supporting the existence of the high temperature arc plasma.
Effects of the air/gas pressure wave expanding and emanating from the point of arc manifestation.
In short manifestation of electrical arcing is akin to the detonation of a ‘bomb’ with the added phenomena that following ignition and proliferation of blast wave, arcing continues given,
a supply of energy to support arc existence
an intimate environment beneficial to support arc existence
What is electrical arcing?
Generally, electrical arcing is the conduction of electrical energy through a gaseous medium, i.e. air.
Air in normal circumstances is a non-conductor of electricity. On the other hand, when air is exposed to intense electrical forces, it becomes a conductor of electricity.
Exposure to intense electrical forces comes about when a suitable electrical charge gradient (voltage) exists between two electrodes. The electrodes generally are metallic conductors of electrical systems, however as in the case of storm lightening, the electrodes are the charge saturated clouds and the body of the earth, ground
A gaseous conductor in free air ‘electrical arcing’, is characterised as a plasma supported between two electrodes and when established emits intense light and is much elevated in temperature (circa 3,000°C to 20,000 °C)
A switch/circuit breaker
The fundamental construction of the rudimentary switch or circuit breaker comprises of two (2) metallic ‘contacts’. When considered in an electrical sense, the two ‘contacts’ constitute a two state system, that is,
When separated an air gap exists between the contacts and the flow of electrical current from one contact to the other is impleaded.
When the ‘contacts’ touch, ‘make contact’, the flow of electrical current from one contact to the other is established.
In either state, the contacts are indeed ‘electrodes’ since a charge potential (voltage) exists between the pair and the magnitude of the charge potential is dependent on the ‘intimate environment’ which exists between the pair of contacts/electrodes.
Theoretically, a switch or circuit breaker is a component that aims to bring about an instance of ‘discontinuity’ in the operation of an electrical system which functions in the continuous time domain.
Discontinuity
Figure 1 Graphical representation of a switch/circuit breaker operating in the continuous time domain exhibiting idealised instances of points of discontinuity
Practically, a discontinuity cannot exist in the continuous time domain since a point of discontinuity attempts to establish that an electro mechanical system such as a switch or circuit breaker exists in two states at the same time instant. In ‘real time’ terms, a short period of time exists referred to as the ‘transition time’, when a switch or circuit breaker transits between the two states.
Consideration of the dynamic operation of the switch or circuit breaker
In consideration of the dynamic operation of the switch or circuit breaker within the transition phase, in either mode of switch operation, opening or closing, the contacts will at some time exists with a small air gap between them and given a suitable supply of electrical energy and the intimate environment about the contacts at the time, the manifestation of an electrical arc may occur.
Closing phase of operation
In respect of the closing phase of operation, given arc manifestation in ideal conditions the existence of the arc should be short lived, since the electrical resistance of the contact/s on closing should be less than arc conductivity. Therefore given that electrical current commutates and conducts through the contacts rather than the arc, the arc extinguishes.
This is all well and good if the physical force of the contacts coming together can overcome the gaseous pressure of the arc which will physically counter and repel the contact closing action of the contacts. Further, given that the contact closing time is fast, such that the high temperature of the arc cannot erode the contacts to the extent that the physical gap between contacts increases rather than decreases.
So it follows that the key parameters in the closing phase of operation of a switch/breaker are the energy of the arc and the energy of the contact closing mechanism which collectively dictate the time period of arc existence. Simply this resolves to a conflict of arc energy -v- contact closing mechanism energy.
The attributes of the conflict can be influenced, logically by the mechanical dynamics of the switch yet also by the attributes of the environment in which the arc plasma is sustained which can be designed to enhance ‘quenching’ of arcing.
Opening phase of operation
In contradistinction, the opening phase of operation of the switch/circuit breaker differs significantly, since as the contact/s physically part an arc can be ‘drawn’ becoming long and thin, such that the arc may still exist when the contacts come to rest at their fully open ‘mechanical end stop’ position.
Further, given that when the switch/breaker is in the closed state, then the parent electrical circuit will be charged with electrical energy, hence any electrical/magnetic energy stored within components of the circuit will decay when the switch/breaker starts to open and in doing so supply ‘stored’ energy to the arc, fuelling and prolonging arc existence.
So working on the premise that in the opening phase of operation an arc is drawn, then to successfully quench the arc, the physical attributes of the arc become paramount, since if the arc is allowed to expand radially even though increasing in length,
Arc conductivity will be increase
The arc will become unstable electrically and also as a gaseous plasma
If significant stored energy exists the energy conveyed by the arc will be sustained
The impact on the surrounding environment of the arc will increase.
However In contradistinction if the arc is long and thin then,
The arc conductivity will be decreased
The arc will become unstable electrically and also as a gaseous plasma
The energy conveyed by the arc will be reduced
The impact on the surrounding environment of the arc will be reduced.
So it follows again that the parameters of concern in the opening phase of the switch/breaker are the energy of the arc, the energy of the contact opening mechanism and the attributes of the environment in which the arc plasma is sustained.
Given the latter discussion it follows that the design objectives of a switch/circuit breaker are most notably to load the scales/balance of conflict, i.e.
Figure 2 Scales/balance of conflict
So in summary, “Why do circuit breakers rely on stored energy to aid opening/closing, in some cases are filled with oil or gas and have internal sacrificial structural features? the answer is to reduce the effects of electrical arcing.
Eur Ing Dr Robert Brown (Robert) is the Executive Director of Fraser George and Associates Limited and is a Consultant Engineer in the fields Electrical Electronic and Control Engineering. Robert is also an accomplished professional Expert Witness having prepared and presented many court compliant reports and presented oral evidence within the High Court, Crown Courts and County Courts.
For further information please contact Robert via;
Email,…robert.brown@frasergeorge.com or robertbrown@robertbrown.uk.com
Tel Land: +44 (0)1777 709175
Tel Mobile: +44 (0) 7976250624