EMC – It can't hurt you, can it?Follow article
Electro Magnetic Compatibility (EMC) is probably the most misunderstood area of product compliance and is often seen as something of a black art (or something out of Harry Potter!).
What is EMC?
The following definition may help: The ability to function correctly without causing undue interference and to be able to operate acceptably when exposed to interference.
Before we explore the subject, it is worth considering the statement in the title of this article. There is plenty of anecdotal evidence of electronic equipment interfering with other systems:
When digital mobile phones were relatively new, it was not uncommon to hear a woodpecker-like noise on the car radio. Phones and car radios are better now but you can still get this effect if you hold a mobile close to a radio. Not dangerous, just annoying.
The airbags of a well-known brand of luxury car were set off when a mobile phone rang close to the airbag sensor. Now we are getting dangerous.
An incident at an EMC test lab some years ago involving a large truck is probably as bad as it gets: radiofrequency interference affected the Electronic Control Units (ECU) and caused the accelerator and brakes to be activated at the same time. Luckily it was on a rolling road and quick thinking technicians killed the engine before the truck tried to escape!
For many years we have been told that all mobiles, laptops and music players must be switched off completely on aeroplanes and only used in flight mode when use is permitted. This is changing, but why now? We will come back to this later.
BBC News Article: Airlines cleared for mobile phone use during flights
When we are considering EMC there are two main modes of interest: Emissions and Immunity.
Emissions – Electromagnetic emissions emanating from a device. These emissions could interfere with communications services or other devices.
Immunity – A device’s immunity to electromagnetic interference caused by other products, radio services or natural phenomena.
Unwanted emissions can be transmitted from an electrical device; through the air – radiated emissions and through connecting cables – conducted emissions. Radiated emissions tend to be at higher frequencies (e.g. 30MHz - 6GHz) and conducted emissions at lower frequencies (e.g. 150kHz – 30MHz).
N.B.: We are talking about unwanted or unintentional emissions here. Intentional emissions such a radio transmitters, microwave ovens and high frequency induction heaters have their own requirements which we will cover in another article.
Regulatory requirements for unwanted emissions are almost universal. No-one wants a product on the market which causes interference with other equipment such as TV’s, computers and mobile phones. This takes on a new meaning when you consider the implications for industrial control systems, computers, cars and aeroplanes!
The ability to withstand interference from electrically noisy sources is called immunity. As with emissions, the interference can be radiated through the air, conducted down cables and by Electro Static Discharge (ESD).
There are a number of sources of interference for which immunity tests have been devised:
- Electric fields and induced radio frequency disturbance from radio transmitters e.g. mobile phones, broadcast transmitters, WiFi.
- Magnetic fields from high voltage transmission lines.
- Pulsed magnetic fields caused by indirect lightning strikes couples into power distribution systems and telecommunications networks.
- Electrical fast transients from inductive load switching.
- Switching of heavy inductive or capacitive loads causing high energy high voltage transients in low voltage (mains) power distribution systems.
- Lightning strikes into HV power distribution systems causing transients in the mains supply systems.
- Variations of mains supply voltage and interference due to power distribution network faults,
- Electro Static Discharge (ESD) by contact or through the air between the equipment and a statically charged object. Man-made materials are often to blame for static, e.g. carpets & fabrics.
These all represent real life conditions to which your system will be exposed.
EU EMC Directive 2014/30/EU came into force on the 18th April 2014. This supersedes 2004/108/EC which can still be used until 20th April 2016. The EU EMC Directive requires all electrical equipment to comply with the generic or product specific standards and to be CE marked. New (non-technical) requirements to improve enforcement of the Directive have been introduced particularly related to traceability and increased responsibilities of the manufacturer, EU representative and distributers.
In Europe, the enforcement of the EMC regulations is patchy, with the exception of Germany most countries tend to be reactive rather than proactive in policing EMC compliance. Having said that we do know of test purchases of some mobile phones by the French authorities. The phones were on sale in supermarkets and some of them were found to have excessive SAR (Specific Absorption Rate) levels outside of legal limits. Also the UK’s Trading Standards have undertaken test purchases and are becoming more vigilant in certain aspects of product compliance. Ensuring that equipment is tested for both emissions and immunity does help the situation, but it relies on everyone playing the game and having their equipment tested!
Another major market is North America and in particular the US. The Federal Communications Commission (FCC) has responsibility for enforcing compliance in the US. The testing can be performed at a competent EMC test lab. However, the FCC actively police EMC compliance and impose large fines on suppliers who place non-compliant products on the market.
The EU requires compliance with both emissions and immunity standards, whereas the US only requires compliance with emissions standards.
N.B. The EU and FCC emissions requirements are not equivalent, although in some cases, e.g. battery powered devices, the EU test results can be used to extrapolate the FCC results.
A product which complies with both EU and US (FCC) EMC requirements will comply with the requirements for 90% of countries where there are EMC regulations, often without further testing.
In Europe the list of standards for compliance with the EMC Directive is published in the Official Journal of the European Union often referred to as the “OJ”. The document which is frequently updated is 22 pages long and lists about 200 standards. OK not all of them are current, but it is still a challenge to identify the right one!
If a product falls within the scope of a product specific standard this should be used because it gives a presumption of conformity (though not the only route for compliance), if there is no product specific standard a generic standard should be applied.
The FCC make life simpler, they incorporate the technical requirements into the legal framework in the Code of Federal Regulations, Title 47, Part 15(b) – Unintentional Regulators, more commonly known as CFR47 Part 15(b).
There are variations in the requirements for some product types, but the approach is generally straightforward and doesn’t involve looking through 200 standards!
Selecting a test laboratory to perform the EMC tests is not as simple as it seems. In the UK alone there are more than twenty companies offering EMC testing. Firstly look for a laboratory which offers expertise in your product type. An automotive or wireless EMC lab is not necessarily the best for a washing machine! The UKAS website is a good place to start:
- Do they offer pre-compliance testing and advice?
- Do they work with the client to determine the optimum test plan to minimise costs without compromising the end results?
- If the product is non-compliant do they stop testing and ask you for instructions?
- Do they offer support to resolve non-compliances?
- How quickly do they deliver the test reports?
Preperation for Testing
Before you start testing ask yourself these questions:
- Is the product representative of the final product?
- Do you have test reports for the power supplies and other sub-systems? This will assist in debugging the system if it fails!
- Can you operate the system in real time to simulate a live installation? The system should be capable of streaming data over any connections to prove that it still works during the immunity tests.
- You may need to provide suitable equipment to connect to any live connections, e.g. Loop-backs, transceivers, loads, GSM connections.
- What if the test results are non-compliant?
The laboratory will often help you to identify the source of the failing emissions which could be a harmonic of a clock frequency or a noisy switched mode power supply. Once you have identified the problem fixing it is may be a simple fix by adding a suitable ferrite to a cable, re-routing internal cabling away from the source of the emission, screening or earthing the metalwork. This is not always the case and you may have to call on the services of an EMC compliance specialist to work out the best solution! A classic example is ventilation holes in a chassis; the size of the hole may allow radiated noise of certain wavelengths out – solution: metal mesh over the hole and don’t forget to earth it solidly to the system earth. The glass door of a microwave oven is a perfect example of this; without the mesh the user would get cooked along with the food!
The same principles apply for immunity related non-compliances, but there is another dimension to immunity. The standards allow for different degrees of immunity depending on the critical nature and use of the product.
There are three categories of immunity:
- Equipment continues to operate normally
- Equipment mis-operates when exposed to interference, but works normally when the interference is removed.
- Equipment mis-operates when exposed to interference, but has to be reset when the interference is removed.
In each case no permanent damage to the equipment is allowed. The forth category where interference causes complete and permanent failure of the equipment is obviously not acceptable!
Which category is acceptable to you will depend on the intended use of the product. If the product is used in a critical application, e.g. medical or alarm system, then the first category would normally apply. Unattended equipment would need to be at least a category 2.
The next issue is the CE + CE ≠ CE scenario. Take two sub-assemblies both of which are fully compliant with the EMC requirements, connect them together in an enclosure and power them from a single EMC compliant power supply. Are they still compliant? The chances are that they aren’t. Individually they are fine but when they are installed in close proximity to each other, problems can occur with intermodulation between oscillators, amplification of power supply noise and earthing circuits becoming radio frequency antennas. So the answer to the CE + CE ≠ CE question is test it and see! Half day in an EMC laboratory will tell you if you need to re-test or use existing reports plus a quick test.
Switched Mode Power Supplies are probably the biggest culprits for emissions failures. Ask the manufacturer for a copy of the current compliant test report for the power supply. If it fails when it has been installed in the product, check the EMC performance by running a stand-alone test on the power supply connected to a representative load. Many battery chargers come fitted with a ferrite in either the mains or secondary (or both) for a good reason!
The converse is also a potential problem. At high frequencies the power supply can behave like a piece of wire! Noise generated in a system is conducted through the power supply back to the mains cord where it is measured as part of the standard test programme.
One of the most critical elements in high density electronic systems is the PCB layout. A short trace can become an antenna to transmit or receive RF noise. As the operating frequencies increase so the behaviour of electronic circuits becomes more like a radio transmitter or receiver with the same problems. Multilayer PCBs allow the designer to include ground planes to conduct RF noise away and to shield the circuitry from interference.
So why can we use our electronics on aeroplanes? The systems on an aeroplane will have been tested to a far higher degree than any consumer electronics. Avionic systems are designed for fault tolerance and immunity to interference to a degree almost unheard of 20 years ago (before everyone had a mobile). The aeroplane manufacturers have taken this into account and have designed their systems to tolerate the mobile phone users who forgot to switch off, about 10% on any given flight apparently. Just as well there is no usable signal at 30,000 feet, an opportunity for a bit of peace and quiet!
Look out for the next article in this series: Wireless devices – a compliance problem!
If you have any questions or would like to find out how this applies to your product, please contact:
Product Approvals Ltd - www.productapprovals.co.uk
Look out for the next article in the series:
Design for Approval - Compliance
Design for Approval - Safety
Design for Approval - EMC Can't Hurt You Can It?