September 02, 2019 by Sam Holland
Every electrical engineer charged with the responsibility of designing and building electrical systems must ensure that they are reliable, safe, and durable. But sometimes, things could go awry: burnt PCBs and components, static shocks, and tripped circuit breakers are just some of the myriad technical issues that come from inadequate circuit protection.
Circuit protection devices help limit the flow of excessive current and voltages (i.e. they offer overcurrent and overvoltage protection), as well as electromagnetic interference (EMI) in electric circuits. We’ll now cover overcurrent, overvoltage, and electrostatic protection for electrical devices accordingly.
When the current flow in a circuit is greater than the current rating of its components, it can cause a multitude of problems. Overcurrent conditions typically result from short-circuits, overloading, or faulty design, and may cause overheating, arcing, electrical fires, and explosions.
Overcurrent protection equipment protects electrical components in a device by automatically opening the circuit when the current rises beyond the maximum rated current of the conductors. Examples include fuses, circuit breakers, and inrush current limiters.
A fuse is a simple overcurrent protection device consisting of a strip of metal wire enclosed in a capsule with two non-polarised terminals. When a current greater than its maximum rating flows through it, the conductor melts, effectively opening the circuit. Fuses come in various shapes, sizes, and current ratings.
Some common examples include:
These fuses are highly sensitive devices that open up very quickly (1 second or less) under overcurrent or abnormal electrical conditions. Fast-acting fuses are suitable for general-purpose applications that do not experience temporary overload currents. They are rated up to 20A.
Typically used in transformers and power system installations, high voltage, aka HV, fuses provide overcurrent protection for devices rated from 1.5kV to 138kV.
These fuses are utilised in automotive applications such as EVs and hybrid (gasoline-electric or diesel-electric) vehicles. Some automotive-grade fuses are qualified by the AEC-Q200 standard. The Automotive Electronics Council (AEC) qualification ensures safety and reliability under high-temperature and mechanical stress conditions, which are common under the hood of modern vehicles.
A close up of circuit breakers in formation. Image courtesy of Pixabay.
A circuit breaker is an automatic electrically-controlled switch that interrupts the flow of high magnitude current. Circuit breakers are more sensitive to overcurrent than fuses and they seldom require replacement. Electrical engineers utilise a variety of circuit breakers, depending on the amperage of the intended system.
A common type of circuit breaker is the miniature circuit breaker (MCB), which protects devices that have a maximum current rating of 100A. MCBs work by automatically switching off the power supply under overload or abnormal electrical conditions. They are commonly used in low voltage applications such as motor drive protection.
Inrush current limiters are safety devices that prevent damage due to the high inrush (instantaneous or surge) current that flows in when an appliance is switched on. They are particularly beneficial in power supplies and DC motors, which can draw up to 300% of their steady-state currents when turned on.
Overvoltage occurs when the voltage flowing through an electrical circuit is greater than the maximum rated voltage of its components. It mostly results from lightning discharge or transient voltage from load switching operations. Overvoltage protection devices (such as lightning arresters mentioned below) help protect sensitive electronic components from voltage spikes, in order to prevent premature failures, arcing, or overheating.
Lightning arresters are overvoltage protection devices that help protect the conductors and insulation of electrical installations from the damaging effects of lightning. It consists of a small lightning rod attached to a surge arrester, which is connected to a deep-underground earth terminal.
These devices are typically placed on the highest peaks of buildings (particularly the roofs of course). When lightning discharges occur close to the installation, the arrester attracts it and gives off a spark, before diverting it down to the earth terminal.
A close up of a circuit breaker. Image courtesy of Pixabay.
Electrostatic discharge (ESD) is a phenomenon wherein a sudden flow of electricity occurs between two charged bodies that come into contact. In electric circuits, it can result from short-circuits, electromagnetic induction, or broken-down insulation. ESD can cause a malfunction or breakdown of electronic components, such as ICs and RF transmitters if adequate protection is not utilised.
Engineers can utilise ESD protection devices to protect electric circuits from the effects of static discharge or stray electromagnetic fields. Below are some ways to achieve ESD protection:
During logistics and storage, anti-static packaging can help to minimise the build-up of static charge and the collection of dust between a device’s electronic components.
ESD-resistant packaging is made from polyethylene terephthalate (PET) or nylon that is impregnated with conductive materials. They work by creating an invisible ‘electric shield’†around the surface of the bag, which protects sensitive components from the EMI that is radiated from nearby electronic circuits during transit.
Varistors (aka voltage-dependent resistors, or VDRs) are electronic components that help protect electric circuits from voltage surges. VDRs have resistances that are dependent on the voltage applied to the circuit.
When the voltage flowing in the circuit exceeds its maximum rated voltage, the varistor suppresses it by reducing its impedance. On the other hand, it acts as a capacitor when the surge voltage is within permissible limits.
A Faraday cage is a type of enclosure comprising a mesh of conductive materials of a specific thickness. Electrical engineers use them to shield sensitive PCB elements (such as ICs and RF antennas) from stray electromagnetic fields. They are utilised in a host of applications, e.g. automobiles, aircraft, USB cables, and microwave ovens.
What we’ve covered above is only a fraction of the circuit protection devices available to electrical engineers. Electrical engineers who use even a handful of such equipment can ensure that the devices they produce are safe, effective, and reliable.
