I can say that electrical short circuit is the nightmare for electricians and us, i.e electrical engineers.
Not only equipment damage that electrical short circuit can cause, but also it could be destructive and can cause fires and human injury or even death.
In my work, short circuit cause complete damage to electrical motors and generators. In many cases the generator catch fire and get completely damaged.
What is an electrical short circuit?
A huge abnormal thermal and electrical power that happens when two energized phases come in touch with each others, or when one phase comes in touch with the neutral or grounding.
Electricity is divided in two, AC and DC source, In DC systems like batteries there are two poles, one is positive(+), and the second is negative(-), These poles must not touch each other directly in any conditions, because if they do, an electrical short circuit will start.
In AC system like synchronous generators and the plugs in your house, the poles are called phases and one neutral, phases and neutral must not touch each others in any case, also different phases must not touch each others too.
Short circuits produce a huge amount of power and heat, which could melt plastic, copper, aluminum and iron, and could cause damage to the equipment and fire.
Why is electrical short circuit dangerous?
A short circuit is dangerous and could cause fire, devices damage or even human injury and death because, it has two effects, Mechanical and thermal effects, i.e produces huge amount of thermal power. Its one of the many shapes of electrical hazard.
When a short circuit condition begins a sudden and dramatic change in the circuit and a large amount of electrical current passes through the electrical system starting from generator passing through cables and devices of the circuit and ending to the fault location.
This current value is in Kilo Ampere which is much larger than normal current.
This kilo ampere current produces high temperature large enough to destroy the faulty device and to melt copper conductors, circuit breakers contacts and almost any thing on its way. This is called, thermal effect of a short circuit.
The mechanical effect can cause flying parts to injury humans around it.
What is A Bad Effect of a Short Circuit?
Short circuits may cause overheating, fire, and explosions, among other things. All of these things have the potential to cause considerable harm and possibly injury.
When a short circuit happens, the system’s current rises to an abnormally high level.
Due to the high current generated by the short circuit, excessive heating occurs, which might end in fire or explosion.
Short-circuits can sometimes take the shape of an arc, causing significant damage to the system.
For example, if an arc on a transmission line is not rapidly cleared, it will severely burn the conductor, causing it to shatter and cause the line to be disconnected for an extended period of time.
Because of the aforesaid negative effects of a short circuit, it is desirable and important to disconnect the faulty portion as soon as possible and restore normal voltage and current levels.
Short circuit has mechanical and thermal effect. Let’s discuss each in more details.
Mechanical effect of short circuit
The current flow during the fault condition in the bus bar has a reverse direction than the normal flow direction. This causes a repel condition between the faulty bus bar phase and the normal one and could cause bolts and other parts to fly away in air in dangerous condition.
Short circuit thermal effect
When current passes through an electrical conductor it causes a rise in temperature. This rise depends on the conductor resistance and the current value.
Th larger the current value the higher the temperature. As mentioned above that the temperature rise is one of two main effects of a short circuit, which are the answer of the famous question, Why is a short circuit dangerous?
If the protection device fails to isolate power of faulty load, Then the current of the short circuit could be very high and produce more and more temperature rise. This rise is enough to cause fire.
What are causes of short circuit?
Many causes of short circuit in all electrical equipment, and in all fields of work, but in general electrical wires, cables and all current carrying conductors by default are isolated or at least are separated from other conductors.
If, for any reason, the different phases in AC circuit get in touch together or get in touch to ground, a short circuit situation occurs.
So, in simple words, the main reason of electrical short circuits is insulation failure between phases or between phases and neutral. But, What makes the insulation to fail?
To understand the main reason of insulation failure, we should first know that the insulation of electrical devices, wires and cables is a layer or multi layers on the conductor, this layer is made of PVC or XLPE and has temperature limits i.e any rise in temperature will cause insulation to fail.
- Now we know that excess heat damage the insulation layer, also mechanical damage and manufacturing issues would weaken the insulation causing it to be damaged.
- Over-current is also one of the main reasons of short circuit because, over-current generates more heat.
- Human faults may be one of the short circuit causes, as any accidental connecting different phases to each others or to neutral will cause a short circuit.
Most common causes of short circuit are mentioned below in more details:
Faulty Circuit or Wire Insulation
Faulty circuit or wire insulation is the first reason because, sometimes cable core becomes damage with the passage of time.
The inner wire material is exposed, due to which old wires allow the hot and neutral wire to touch each others, this causes a short circuit condition.
The life span for a cable is up to forty years. Another reason is, sometimes fault occurs in a circuit. Sometimes faulty appliances like faulty power cord, faulty plug, or a flaw cause short.
Faulty Appliance
If we plug an electrical appliance in the plug. If there is a fault in a device the current flows can cause a short circuit immediately. For instance, Wet appliance can cause a short circuit.
Human faults
Mistakes happen! don’t they? I believe that the most dangerous mistakes are caused by expert electricians and electrical engineers who work with great confidence.
Some times humans make mistakes while working on electricity and power cables.
Some mistakes are because some electricians are not following electrical safety procedures and electrical isolation and LOTO system.
These mistakes could cause electrical connection between two different phases causing electrical short.
Using not isolated hand tools could accidentally connect tow phases directly causing a short.
So, its important for each electrician to apply proper isolation lockout and tag out (LOTO) system. And also use proper isolated hand tools.
I’ve written a detailed article about LOTO isolation you can read it for more information.
Types of short circuit
- Phase to phase, when two energized phases come in touch.
- Phase to neutral, when one phase touches the neutral line.
- phase to ground, when one phase comes in touch with a grounded part.
Phase to phase short circuit is the most dangerous of all types.
Protection against electrical short circuit
As mentioned above a short circuit is dangerous and must be taken into account when designing any electrical system.
The protection devices are fuses, circuit breakers and advanced protection relays.
Fuses and circuit breakers are mostly used. While protection relays are used in industrial loads and switch-gears.
Before failure
Keeping electrical devices, motors, transformers, and equipment safe needs a good preventive maintenance program and periodic electrical inspection.
In this electrical maintenance program, you as an electrical engineer or electricians should take periodic readings for temperature and insulation resistance, and compare them with the manufacturer recommendations.
Keep electrical equipment dry and away of water sources. Use weather proof equipment for outdoor use.
Train your staff
All electricians should be well trained to apply proper isolation lockout and tag out (LOTO) system.
Also use proper isolated hand tools. This way we could prevent and avoid serious faults and human injury.
I’ve written a detailed article about How to ensure hand tools safety?, on my other safety site, you can read it for more information.
During failure
Electrical protection devices are so smart today, and could detect a short circuit of hundred and thousands of amperes, and are apple to isolate and stop the defected device and protect it before the short circuit cause total damage and fire.
DC and AC fuses, miniature circuit breakers (MCB’s), molded case circuit breakers (MCCB’s), and advanced protection relays are used to protect electrical panels, motors, transformers, and also switch-gears against any possible short circuits.
What to do in case of short circuit?
If you are not an electrician or electrical engineer. Then, dealing with a short circuit need to be focused.
If a circuit breaker trips instantly this is a strong indication that the circuit breaker tripping is due to a short in the circuit not due to overload.
Follow the below steps in case of your breaker trips instantly.
- Switch off all devices and appliances which are affected by the tripped breaker.
- Check your electricity box and find the tripped breaker. It should be on the off position.
- Switch on the circuit breaker.
- Turn on all devices one by one. The faulty device will cause the breaker to trip instantly as you turn it on.
- Call electrical inspection service to check this device for you.
If you are an electrician, you should isolate the faulty device and reconnect power. Keep in mind that the faulty part may be a wire in the circuit.
You better isolate all devices which are powered by the tripped breaker, turn on the breaker if it trips again, a wire should be damaged.
Disconnect main circuit breaker and check wires by multi-meter one by one to find the faulty one.
Can A Short Circuit Damage a Breaker?
Of course, A short circuit can badly damage the circuit breaker because of its thermal and mechanical effect.
You may ask, how come a short circuit destroy the device that should stop the short circuit?
Circuit breakers are built to shut in the event of an overload or short circuit. However, there’s a possibility that they may fail without triggering if the short circuit current is too high, which could cause more electric damage.
Short circuit currents are too large, and able to make huge damage to any electrical circuit devices, the value of this current depends on many things like the load power, voltage level and circuit design, the main job of the protection device is to isolate the damaged part of the circuit as fast as possible.
Each protection device is designed to handle a certain short circuit current value, if and only if, the fault current is less than or equal to this value, then the device will be able to isolate the faulty device, but if the current value is larger than the designed one, then the protection device it self will get damaged.
Short circuit case study at my work
The image below is in my workplace. A damaged synchronous generator due to a short circuit.
The winding of the generator was short circuited and completely damaged as fire started inside it. The main cause of this short circuit was the moisture.
In a foggy day, the moisture entered into the windings, causing the insulation to decrease and the short circuit to happen.
You may ask, How can the moisture enter the generators? Well, its all about the IP rating of the generator.
Simply, its not suitable for working outdoors. It’s somebody’s mistake when ordering the generator.
I really don’t know who is this guy, I mean the guy who made the purchase order of low rating IP generators for outdoors working. The case is the case. We have to deal with this situation one way or another.
What Happen to Current and voltage in Short Circuit?
The short circuit current goes to kilo Amps values. These values are destructive.
Because the source of electrical current has been diverted from its regular flow direction, it encounters almost no resistance during a short circuit, Of course, in the real world, the resistance doesn’t go to zero.
As a result, excess heat is produced, which can damage electric equipment. If a fuse is present in the supply circuit, it will blow out, opening the circuit and interrupting the current flow.
According to a fundamental electrical theorem, the short circuit current is determined by critical parameter:
According to Ohm’s law
V=IR
The load is bypassed when there is a short circuit, and resistance becomes zero.
Then I =V/0;
As a result, the current in case of a short circuit is ideally infinite, It’s just math.
Of course, R is not zero, there is still considerable resistance in the wire between the voltage source and the point of failure.
So yeah, current doesn’t really go infinite, the short circuit current goes to kilo Amps values, the power source provides as much current as it can.
We may State that a large quantity of current travels through the circuit. How high is it? This is depends on the voltage level at which the short circuit occurred, the distance between the problem and the source, and the cable used in the wiring.
What Happens to Voltage in Short Circuit?
Throughout a short-circuit circumstance, the voltage in between two points in the circuit excluding the source turns ZERO, and the reason for this is that the load becomes zero, or the circuit is not linked to any form of load. As a result, it draws a massive amount of current, and the voltage drops to ZERO.
What is difference Between Short Circuits and Open Circuits?
From the names, open and short, the open circuit is the case of having no current flow and no connected load between the voltage source.
While a short circuit is the opposite case, the voltage terminals of the power source are connected to each others with very low resistance, not zero but almost zero, the flow of current is in kilo ampere in a path that is not intended for current flow.
What is Difference Between Short Circuit and Ground Fault?
A ground fault is a type of the short circuit, it means that a phase is short circuited to the ground. It also refers as a phase to ground short circuit.
In some cases the value of the ground fault current is not as much as kilo amps of a short circuit, it may be in amps and of course may be in Kilo Amps as well.
The reason of this, is that the ground fault may occur when the insulation of an electrical equipment, like a motor, has a weak point that passes current to the motor body.
A ground fault relay can detect the value of the fault current and protect the circuit, but you should search about the faulty load if the circuit has many loads.
You can isolate all loads and reconnect them one by one to find out the faulty load.
On the other hand, the short circuit current is high enough to trip, or destroy, the protection device.
A ground fault circuit interrupter, also refers as GFCI, is used for protection against ground fault. I’ve written a detailed article about GFCI you can read it for more information.
Does Grounding Prevent a Short Circuit?
Grounding is not a way to prevent the short circuit from happening. It is a way to recognize a short circuit and make a safer path of the fault current to ground rather than a human body.
Earthing is generally used to stop the harm to our bodies and domestic appliances. When the voltage goes through electrical wires, our appliances may be damaged.
Read my article about grounding for more information.
How short circuit blow a fuse?
The large value of current passing through the fuse during the short circuit, makes its element to heat up and melt down.
The element of the fuse is designed for currents much lower than the short circuit currents.
This value depends on Its material and cross sectional area. The thermal effect of the short circuit current is capable of blow the fuse element so easily.
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