Can you vandalise an anti-vandal switch?
Writing blogs, especially on technical subjects, is a strange way to make a living. What is even stranger is when blogs start to write themselves.
When I started researching and preparing for this blog, my intention was to discuss a new range of vandal resistant switches. With my interest in applications, materials and performance, I was fascinated to understand exactly what 'vandal resistance' actually means in the world of switches.
As we've discussed in previous blogs, we use established standards and ratings as a convenient shorthand to identify how products will perform under certain conditions and to help us choose products for specific applications.
I have been using the IP ratings for much of my professional career to provide my customers with information about how certain connectors will perform, and more recently I have been using the equivalent IK ratings too. In all that time, I had never actually witnessed these tests taking place.
I also felt that it was important to understand the information that IP and IK ratings do not provide as much as the information that they do.
IP ratings are very familiar. They provide very clear information about the conditions under which a product is resistant to the ingress of water, all the way from a light sprinkling to full immersion. However, in order to provide a meaningful comparison, the tests need to be conducted under controlled conditions. It simply isn't good enough to spray your switch with a garden hose or dunk your connector into a bucket. It's just not very scientific.
The use of IK ratings requires the same scientific rigour. These ratings provide an indication of how much impact force a product can resist whilst remaining functional. It is important to remember that this does not guarantee that a device is vandal proof.
At the beginning of March, we visited the UK headquarters of APEM Components, a major manufacturer of switches and indicators.APEM boast a test laboratory to allow them to test their own products. APEM was kind enough to allow us access to their test equipment to see these tests in progress.
However, in the course of the visit and our tests, things did not go according to plan, and taught us more than we had planned to learn!
We began our tests with the familiar IP ratings, something that I have wanted to do for ages. The IP67 test involves the immersion of the test subject under 1 metre of water and leaving it there for a fixed period of time. In preparation for this test, I had obtained an IP-rated box from Fibox (119-5531), one of our enclosure suppliers, and into the transparent top cover drilled a hole according to the instruction manual - for a 19mm switch the hole must be 19.2mm in diameter. This was a relatively easy task with the help of a pillar drill, a stepped drill bit (023-2513), a hand reamer (045-6894) and a set of digital calipers (841-2518). Suitably prepared, the 19mm switch was placed into the hole and the rear mounting nut tightened.
Rather than find a bucket that is 1 metre deep, APEM has an IP-testing tank. The suitably prepared enclosure was screwed to the base of the tank, which was then filled to a depth of 1 metre. Constructed of aluminium and perspex, it is possible to observe the test subject in situ. However, the test requires that the test is conducted over a fixed time period. So, while we turned our attention to other experiments, our test subject was left to soak quietly in the corner.
The IK rating of a device is determined by applying a known force to the test piece and observing the results. In order to make the results consistent and controllable, the test uses a pendulum of a fixed length and a 5kg mass, with different forces applied by dropping the mass from a known height.
For our first test subject, we chose APEM's AV09 series of pushbutton switches in a 19mm version. These switches are manufactured with a stainless steel case and are front-panel mounted for a wide variety of applications - including security. We used a switch rated to IK08, the test for which translates as an impact force of 5J. Looking at the documentation, this test is conducted by mounting the switch on a vertical panel and dropping a 1.7kg mass from a height of 0.295 metres.
The same test can be performed with a 5kg mass dropped from 0.1 metres. We conducted the tests 7 or 8 times, and the results were impressive. Despite the pounding, there was no damage to the switch and in fact no marks at all.
Emboldened by the results of our first test, we moved onto the larger 22mm switch from the same AV09 family. The technical documentation states that this switch is rated to IK10. This translates into a force of 20J, which for the purposes of our test was to be achieved by lifting our 5kg mass to a height of 40cm.
This is where it all went wrong. Basking in the warm glow of our successful test, we bolted the 22mm switch to the fixed plate that was to be bolted onto the test rig. Without a thought, we lifted the mass up to 40cm, let it fly, and inspected the results.
We’d broken the switch.
A certain amount of panic then set in. Had we applied too much force, or worse, had we proven that APEM's documentation was wrong? What had happened?
To find the answer, we returned to the documentation and looked carefully at the installation instructions. In order to obtain the best performance from the switch, there are specific instructions concerning the mounting hole, the placement of the sealing on the O-ring and the correct torque to be applied to the mounting nut.In our haste, we had not taken the time to understand the correct installation procedure and therefore not mounted the switch correctly. We had fallen foul of the oldest schoolboy error in the book. We had not read the instructions!
With these results in mind, we returned to our switch that had been soaking in the corner. At first glance, the test appeared to be successful. However, as we found out, the results of an IP test can be subtle and require close observation. Once we looked closely, we were able to see a very slow drip of water forming around the outside of the switch. With this in mind, we drained the test chamber and carefully dried the outside of the test subject to remove any excess water. We then removed the transparent lid and found a small amount of water had made its way into the box.
Over the course of a 30-minute test, about a teaspoon (5ml) of water had leaked into the box. It is important to consider the conditions of the test and their importance. The depth (and therefore the pressure) of the water had revealed a weakness in the seal, a weakness that only revealed itself over a period of time.
Having already learnt a lesson over the course of our IK testing, we approached the result analytically. We decided to re-run the test, paying close attention to the installation of the switch. In particular, we made sure that the mounting nut was tightened to the correct torque setting. We then fixed the box and immersed it as before.
Returning after a quick lunch, we were happy (and not a little relieved) to find that the test had been completely successful. Not a drop of water had leaked into the box.
It was clear to me that we had learnt a very important lesson from our experiments. Tests fail when products are incorrectly installed. Equally important is the understanding of what that may mean in the real world.
Tests conducted in the laboratory are great. They prove what a device is capable of under certain conditions, but more importantly, there is nothing at stake rather than a failed test.
In the field, however, a failed installation is far more serious. A failed installation is an expensive problem. A failed installation means returned equipment, repairs and replacements. It means lost time, lost money, lost reputation.
The choice of components for a particular design is, obviously, of desperate importance. However, as both of our experiments have proven, the correct choice of component is only part of the solution. The best piece of equipment in the world, manufactured using the finest components, is of little value if those components are not installed correctly.
And so, as with some of the greatest experiments, my investigation of vandal resistant products led to a much more important lesson.
Read the manual!
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