EMC Confidence Checking
06 Jan 2021
Author: Keith Armstrong
EMC Confidence Checking
I was recently working with a manufacturer of stand-alone ticket machines, who had big problems with Ethernet-related emissions. Long story short – it was the Ethernet hub they had incorporated in their machine.
They had assumed that all CE-marked Ethernet hubs would be equally good for EMC, and they had several different types lying around their design department. We tested them all individually and they were all pretty good except for one type which was appallingly noisy. Of course, this was the type they had specified for manufacture, not knowing its EMC was rubbish!
Digital electronics with high levels of emissions at certain frequencies are also lacking in immunity at those same frequencies. The worse the emissions, the worse the immunity.
However, standardising on a different make/type of hub would not guarantee lack of EMI problems in the field. This is because most commodity electronics with brand names that we recognise is simply cheap stuff with a 'brand name' label applied.
Most of the brands we recognise don't bother to check the EMC (or the safety) of each batch of cheap stuff that they have delivered, and it might be that the batch causing the ticket machine manufacturer's EMC problems was caused by a bad component and wouldn't be repeated on subsequent batches.
Likewise, future batches of the other types of Ethernet hubs that passed our tests in their workshops, might suffer comparable EMC problems due to a bad batch from their cheap stuff suppliers. So changing the type/brand of Ethernet hub to one that didn’t have EMC problems at the moment, could not be a future-proof solution.
The only practical solution to avoid having EMI issues with their (very expensive) ticket machines due to a (very cheap) Ethernet hub, was to either check the EMC of each batch of hubs they bought, or checked the EMC of each ticket machine. Of course, checking the batches of hubs avoids the problem of only finding the problem after adding a lot of value.
EMC checks are quick and easy to do with spectrum analysers and D-I-Y RF cable-current-monitor probes, with an overall cost of around £1k. They are easy to use, too, even by someone with no technical abilities if a technical person sets-up the spectrum analyser and writes the ‘checking procedures’.
Although the case study above concerned a low-cost 'commodity' electronic device, the exact same issues apply to all purchased electronic devices, and this widespread problem was the reason why the EU Single Market was upgraded to "Version 2" in 2016 with the release of a large number of replacement Directives.
The big change in V2 of the EU Single Market was that all of the companies within a supply chain now bear the responsibility for only supplying products that really do comply with all of the EU Directives that they are supposed to!
So an alternative to checking the safety and EMC of all of the electronic products/devices we purchase, before incorporating them into our own products, is to ensure that their supply chains guarantee that the purchased products are compliant.
This is a job for our QA inspectors, and one of the things they should look for when assessing anyone in a supply chain, is whether they use quick and low-cost EMC checks like those I described above.
It is all very well relying on test certificates from approved EMC test labs, but how many items from each batch get fully-tested in such a test lab? Because such lab tests are very costly and time-consuming, perhaps only one item from a batch might be tested. Sometimes several batches might only have one sample tested, because of the cost and delay. Such infrequent testing can't provide us with a great deal of confidence, which is why we like to see quick low-cost EMC checks done as well. We might call them EMC confidence checks.
On high-cost purchased products, it would not be unreasonable to expect to see 100% of them being "EMC-checked" by suppliers, distributors or agents, or to perform such quick EMC confidence-checks ourselves on every one before putting them into Stores for Production to use.
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