We’ve had some problems with RFI transmissions from nearby buildings that have disturbed signals in our CCTV system – this is a problem for us as we cannot lose situational awareness.
We have been working at tightening up our site’s ability to handle rogue RF transmissions but we’re wrestling with how to actually measure how tough the CCTV system has now become. Would an ESD test give us a good idea of how capable our defences are? What sort of equipment would be needed to conduct such a test and how would we conduct it?
A: It’s not uncommon for the low power signals on security LANs to be messed up by strong RF signals. It’s likely any disturbance from a nearby building would be coming from a direct RF link mounted on the roof – those signals would need to be fairly strong to give you serious trouble so it’s possible the problem is inside your own building. The electrostatic discharge test you’re talking about would certainly give you an idea of your system’s ability to resist RFI.
ESD tests are a pretty serious affair and involve thumping your equipment and peripherals with a spectrum of 320MHz on a rise time of 1 nanosecond. An ESD test is going to be socking your shields and highlighting flaws in PCB layout, not just shaking up the cable plant. This means you need to be prepared to measure impact on these peripherals to make the ESD test worthwhile. Equipment you’ll need will be an ESD generator with HV relay, internal resistance of 330 ohms and capacitance of 150 pF. You’ll want your calibrated load rise time on this unit to be 1 ns. If you decide you’re going to test elements of the system for indirect electrostatic discharge, then a field coupling plate will also be required.
When you’re setting up the equipment for the test make sure you pay special attention to decoupling and immunity levels of auxiliary equipment. The last thing you want to do is go tearing through expensive CCTV equipment. And when you’re looking around for somewhere to put the simulator ground lead, remember to attach it to the ground plane on the test side of the equipment being tested.
Without going into great detail, the ESD test procedure demands you first decide whether you’re going to test for indirect or direct discharge. You then need to mark the discharge points on the equipment being tested. It’s important to start the procedure down at 2 kilovolts and then use 5-20 pulses per second as you roll through a general survey of the discharge points. Having completed this process, you then step up the charge level to 4kV or 8kV (it could be 15kV in air).
This last procedure will depend on whether there’s an ESD threshold you’ve set the equipment up to endure. As mentioned before, you don’t want to hammer an unprotected system with a 15kV blast. Test to the level you’ve protected the system to, allowing a kV or so extra in the procedure to give the system room to survive. If your equipment picks up any failures during the test process, then you’ll need to approach these failure sites in single shot mode and re-test them to establish exactly what’s going on.
Make sure you run through and record the results of at least 50 test shots to ensure you’re getting consistent failure responses to the charges you’re introducing. ESD testing is tough and you may need to average out the responses to your tests in order to establish prevailing system conditions – a proper recording process is pivotal. This will allow you to go back after the procedure is completed and implement appropriate fixes. It’s very smart to draw up a good-sized map of the system you’re testing. Make it big enough to allow copious notes to be made as you comment on progress (or lack of progress) in resolving the problem.
It’s very unlikely that any fix is going to work first time so there’s going to be trial and error involved. The kind of fixes you’ll be looking at include improved bonding of switches and keys, paring down seam gasketing, improvements to bonding of panels and doors, as well as better shielding of input/output cables and shielded connectors. When you’re building up a series of fixes, be sure to leave each fix in place even if your progressive tests reveal the latest action you’ve taken didn’t help. Instead reverse your thinking. The idea is to build up your fixes till the problem is solved then remove them one at a time, testing to see which failed.
Having said all this, it’s hard not to think your best and cheapest option is going to be to take the problem links to fibre.
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