Date: April 29, 2021
In the real world, electronics can be subject to all sorts of nasty electromagnetic events. These 'transient' events could damage or destroy devices unless careful consideration has been given in the design stage to integrating protections. Large transient energies caused by lightning strikes (aka surges), electrostatic discharge (aka ESD), Electrical Fast Transients (EFT), and other electromagnetic phenomenon including can all damage electronic devices. These energies can be introduced through ports or anywhere a user can physically touch the electronic device.
Devices which have been damaged by environmental transients are often difficult to diagnose in the field. The device may appear to have stopped working or it may exhibit erratic behaviour.
The measure of a device's robustness to transients is it’s Electro-Magnetic Compatibility (EMC). This is a measure of how a device interacts with it's electromagnetic environment, and with other equipment. Most markets legally require that your device be EMC tested in a lab to prove it is compliant with their EMC requirements before it can be sold. Failing this testing is expensive as this requires redesign and retesting late in the project.
Many EMC problems are not simple or obvious, so they must be considered at the start of the product design. Leaving these considerations to the end of the design cycle can lead to overruns in the engineering budget and schedule.
See our previous article on compliance for more general EMC information - Electromagnetic Compatibility (EMC) Compliance - Answers to Frequently Asked Questions.
This article specifically addresses how lightning induced transients (surges) can damage your device even if there was no ‘direct’ strike.
The most obvious way in which lightning destroys electronic devices is, of course, by a direct strike. These will almost vaporize anything and as such there aren’t many practical ways that a device could be protected from it.
The image below shows a household electrical distribution box which has been destroyed by a direct strike.
Figure 1: A household electrical distribution box which has been destroyed by a direct lightning strike.
Thankfully direct Strikes are very uncommon but there are also indirect effects from nearby lightning strikes which can still wreak havoc. It is actually possible to protect against the damage these indirect effects can cause as their energy is magnitudes lower than a direct strike.
These indirect effects are much more common than direct strikes because the effective radius of a lightning strike is so much larger than the small area the lightning directly strikes. A strike could indirectly damage electronics even hundreds of meters away.
The image below shows a product’s circuit board/PCB which had components partially vaporized by indirect lightning effects. There’s a damaged shield bonding capacitor (these are normally rated for 1-2kV) and a damaged ethernet transceiver IC. The pressure from the chip being vaporised internally was enough to blow off part of the encapsulation compound!
Figure 2: An example of a product which had components partially vaporized by indirect lightning effects.
The effective radius of a strike can be hundreds of meters because of the incredible power that they contain. The average lightning strike contains, on average, approximately 1 billion joules of energy. This sounds like a lot of energy but, to give this some perspective, this is ‘only’ enough energy to boil ~24 cups of coffee. Lightning is so destructive because this modest amount of energy is delivered in just milliseconds or even microseconds. 1 billion joules over 1 millisecond is
1 trillion watts of power.
Even just a small portion of this energy coupled into a device is enough to cause damage.
There are two mechanisms that allow energy from a strike to couple into a device indirectly. These are:
Figure 3: Graphic illustration of electromagnetic pulses propagating outwards from a lightning strike.
The incredible voltages and currents of lightning strikes produces electromagnetic pulses (EMPs) which propagate outwards (at the speed of light) in electromagnetic waves. The magnitude of these waves can be so high that they couple or induce significant current and energy into nearby conductors (e.g. power lines or communication cables). The coupled current can be on the order of hundreds to thousands of amps.
There are two simple factors which gauge how much energy is transferred:
It’s important to note that buildings and the materials they’re made from do little to impede or attenuate EMPs. Cable runs are almost as susceptible inside as they are outside. Dangerous energy can couple in regardless.
This coupling mechanism works through the interaction of lightning strikes and the earth (e.g. dirt, soil, etc.) around impact points.
Soil has a finite resistance and therefore the earth is literally a giant resistor. The current from the strike on earth travels outwards from that point. This current produces voltage potentials in the earth's surface as it travels out that then drops off in magnitude as the current spreads out and the current density reduces. Ohm's law states that where there is current flowing through a resistive material that there must also be a voltage potential. For example - the voltage potential between two points on the earth, in proximity to a strike, could be tens of thousands of voltages different due to the charge flowing through the ground.
Systems which have multiple connections to earth (in different locations) can be susceptible to energy coupling through those grounding points from ground potential gradients induced by lightning currents.
One example of a common system which can be susceptible to this coupling mechanism is an ethernet cable that spans from one building to another building (e.g. the network of a university campus). Each of these buildings will have their own ground rod dug into the earth which the neutral and earth conductors of the building's wiring system are tied to. Therefore the electrical devices in that building are roughly at the same potential as the building’s earth rod. During the instance of a lighting strike in close proximity, these two buildings could be at potentials thousands or tens of thousands of volts different. This might not otherwise have been an issue if it wasn’t for the ethernet cable that connects the two buildings together. This cable would have equipment at either end which are at very different voltages. Ethernet is normally designed to be robust to these events through galvanic isolation, but it will still fail if the voltage potential is great enough to breakdown the isolation.
The image below illustrates this coupling mechanism.
Figure 4: Graphic illustration of ground potential gradient coupling.
There are various components that designers can choose to protect their electronic products from lightning surges. The exact method implemented and which protection components are selected depends on the project and technical constraints.
Here are several categories of transient protection components which a designer can pick and choose from.
TVSs are semiconductor devices designed to provide protection against transients by limiting voltage. They are designed to operate in the Avalanche mode and essentially clamp to a set voltage threshold.
Crowbar devices conduct when the voltage over them reaches a threshold which causes them to trigger to an on-state. In this state the crowbar devices limits the voltage all the way down to approximately zero. Almost like a metal crowbar was placed across the conductors (hence the name)!
This is just a fancy way of saying that there is an ‘air-gap’ in the device or port which prevents a transient surge from being able to conduct to/from earth.
Galvanic isolation protects devices because transients' currents are prevented from ever entering the electronic device. Electricity can’t directly jump the gap without significant potential.
Figure 5: Graphic illustration of an isolation component preventing current from flowing during a ground potential transient.
Here are a few examples of common components which designers use to galvanically isolate power or data ports:
The design below demonstrates how a designer might go about protecting a RS485 transceiver which could be attached to hundreds of meters of cable. The solution is robust to high high energy surge, EFT, and ESD transients.
Figure 6: Image of RS485 protection components on a Bourns development board. [2]
Figure 7: Symbolic representation of the above PCB design. [3]
The circuit design was designed with three different transient protection components:
RS485 transceivers are inherently tolerant to -7/+12V inputs, hence the choice of these TVS values are just above these values.
These parts all work in unison to make the RS485 bus incredibly robust to almost any transient which in turn improves reliability.
A designer can only hope to protect their electronic product until they have gained a fundamental understanding of the principles by which transients can be generated and damage their designs.
Lightning strikes can damage devices even if they do not experience a direct strike. The potential damage radius of a lightning strike can be hundreds of meters.
Designing for lightning and other high-voltage electrical transients can seem like a daunting task at first, but there are common, conventional, and proven methods for protecting electronic circuits from these events.
At Beta Solutions, our experienced electronics hardware engineers have the skill and proven track record to use the best suited design methodology to protect our clients' products from the transients they may ever experience. You can get in touch with us to discuss any idea you have in mind, or a problem you may need solved, via our contact page or give us a call.