What Exactly Is a Short Circuit

I  resumed my shift, then proceeded to check my email and read the report of the previous shift. They wrote about a breaker tripping continually, stating that necessary checks have been carried out and they figured the circuit breaker was fault. They also advised to reset the breaker anytime it trips until a replacement is made available. 

An hour into my work shift it tripped again and I decided to carry out my own checks then I came to the conclusion that the root cause of the tripping was a short circuit. 
Due to Insulation degradation, a short happens intermittently and anytime it happens, the breaker trips. When you flip the breaker on again it holds but after a while sufficient current jumps through the cracked insulation to form a conducting path between red phase and yellow phase and it trips. 
Enough of the back story, I reported to my boss stating that a short circuit was responsible for the continual tripping and the circuit breaker wasn't faulty then the question he asked was "What exactly do you mean by the term short circuit? "   Was he testing my knowledge? I don't know but I answered him and i also got the motivation to write this article.

A Short Circuit is a fault condition in an electrical circuit where two parts of a circuit that should not come in contact with each other eventually come into contact. It could be due to poor wiring, insulation failure, dust, carbon deposits build-up or a foreign body like a gecko e.t.c

 When there is a short, it tends to trip a breaker, blow a fuse or cause an Arc Flash. Short circuits are classified as Bolted Shorts, Momentary Shorts, Intermittent shorts, or High-impedance short.

What happens during a short circuit

Under normal conditions, the flow of current in the circuit is limited by load impedance but electric current tends to flow through the path of least resistance, and this is what happens during a short circuit condition.  An unintentional low resistance path between two points in a circuit is created and current takes a short-cut bypassing the normal route.

Let us bear in mind that V is analogous to pressure, and I is analogous to flow rate.

According to the formula I=VRI = \frac{V}{R}, when the resistance (RR) to the flow of current disappears suddenly while the same magnitude voltage (VV) remains to push current around the circuit, a massive amount of current (II) will flow due to the decreased resistance. This sudden surge in current can generate a tremendous amount of heat, which is enough to blow fuses, cause an arc flash, damage electronic components like your motherboard, and potentially destroy the power source.

How does a short circuit destroy the power source?

A short circuit condition affects the power source as it forces the power source to provide more power than it is designed to deliver. How?, As we all know P = IV. A 330 W power source designed to output 110v would deliver a maximum of 3 Amps current only.
 
We also know that  I = V/R.  that is 3 Amps of current was flowing, 110 V and a circuit resistance of 36.67 ohms in normal conditions. Suddenly there is a short circuit caused by a wall gecko. This gecko has a resistance of 2 ohms based on I = V/R this would cause 55 amps to flow immediately. 

The power source cannot deliver more than 3A at 110Volts so as the short circuit tries to pull 55 Amps, Voltage would try to drop to compensate for the current being pulled but the power source would also heat up and get destroyed assuming there is no protection (fuse or breaker) for the power source.

  • ·         Heavy current that flows due to short-circuit causes excessive heating which is the cause of arc, fire, or explosion you see during a short.
  • ·         Voltage always tends to drop when there is a short circuit, it sometimes reduced to zero,
  • It takes milliseconds for everything explained in this article to happen. 
The explanation here may have been oversimplified and, I hope it doesn't offend anyone

Comments

Post a Comment

Popular posts from this blog

How to check continuity over long distance

Image
When I began my career as an electrical technician my first challenge was the problem of checking the continuity of a wire that spans a long distance. My problem was compounded due to the physical layout of the wire under test. Let me use a simple picture to layout the nature of my problem. A bunch of Armoured cables outgoing from the  PCC (Power Control Center) in Building 1 feeds a group of distribution panels in Building 2 over 100ft away. Ideally, the cables should be tagged but due to aging and poor maintenance, most of the tags were not visible.  The problem was that some Distribution boards in building 2 had power while some others didn't.  The DB of interest didn't have power and thorough checks showed that there was no power on the cable feeding that DB.   All outgoing cables from Building one PCC had power so I suspected that the cable was no longer continuous. Anyway in the initial design of the facility 3 spare cables was laid underground but without tag
Read more

Making sense of a transformer nameplate data

Image
The nameplate of a power transformer contains the following details as per standard, then additional information could be provided varying from manufacturer to manufacturer. Name Of Manufacturer Serial Number Year Of Manufacture Connection Symbol – This tells you the HV winding and LV winding configuration of a transformer and difference in phase angle between them. Example DYN11 where the first letter represents the HV winding and indicates that it is connected in delta, the second letter represents the LV winding and indicates that it is connected in Star(wye)  and the third letter N indicates that the LV winding has a Neutral while 11 denotes a 30 degree lead in phase angle. This article explains more on the Vector group of a transformer and why it is important.  (Transformers connected in parallel must have the same vector group i.e. same phase angle shift to avoid circulating currents. This is a situation where one source will become load to the other sourc
Read more

Stall Rotor Protection Using Magnetic Overload Relays Oil Dashpot type

Image
  A Magnetic Overload relay works by detecting the magnetic field strength generated by the current flowing to the motor.  In most cases the relay input is a representation of the  current (through C.Ts)  flowing into the motor.   The Magnetic Overload Relay is built in such a way that the magnetic flux set up by current flowing through the  coil  drags (pulls) the core upward. A typical dashpot type magnetic overload relay is shown in the image below.                                    Construction. The relay consists of two main parts which is the trip mechanism at the top and the dashpot at the bottom.  The parts of the relay are further divided into a coil, a plunger (core), an oil-filled dashpot and switching contacts (trip contacts). The plunger is attached to a piston (disc). The piston is suspended in an oil filled chamber.  The relay coil is connected in series with the motor supply circuit. During normal operation magnetic flux induced by the coil is not great enough to c
Read more