Xavi Salada - Techna International Ltd
Dr. Kenneth Tong - University College London
John Mestitz - Techna International Ltd
Circuit breakers are an essential safety feature of every building—key to preventing fires and protecting people and their belongings. Such devices must pass a number of safety tests before distribution. Electrical tests ensure the proper functionality of the device and mechanical tests guarantee the mechanisms can operate repeatedly for a long period of time. Completing these tasks manually is labour intensive and time consuming as tests are run in large quantities and can last for hours.
Techna International designs and manufactures a wide variety of electric safety devices targeting industries and residential homes, including circuit breakers such as miniature circuit breakers and residual-current devices. Before selling a product, it needs to be tested to guarantee quality, especially a device used for safeguarding electrical equipment. Manually testing a large number of devices is unviable for many reasons. Constant attention from a human operator on that scale is practically impossible, human error threatens the homogeneity of the test results, and working closely with the devices is very dangerous due to the hundreds or even thousands of amps of electrical current used in the testing process.
We needed an automated test machine to make it safer to test new products in the development stage and practical to test a large a quantity of devices during production. With this automatic system, an operator can control the test machine through a computer program instead of being in direct contact with the devices.
The test machine consists of two sections. The first section includes a pair of pneumatic pistons used to power cycle each unit under test (UUT) a number of times specified by the operator, as well as sensors to determine current levels and pin position. The second section has the UUT connected in parallel to a relay that acts as a bypass in case the device trips. A voltage sensor determines whether the UUT is still engaged or malfunctioning because of fatigue. LEDs indicate the test pass status for each power cycle. We repeat this schema using a programmable power supply to increase the number of devices that we can simultaneously test.
Running the test with the circuit breakers arranged in series runs the risk that some of the devices may trip due to heat generated by the amount of current passing through them. In this case the system needs to recover and activate the specific relay to bypass the tripped device so the test can continue. To do that, the program continuously scans all voltage sensors placed after each device to determine which UUT has tripped. When the program identifies the tripped UUT, it stops the power supply, activates the appropriate relay, and switches an LED to display this to an operator. The system then resets the power supply and the test continues. On the front panel, the operator can view the status of each UUT, the current used, and the remaining time for the test.
Control of the system, including the pistons, relays, LEDs, and binary sensors, requires a large number of digital inputs provided by the NI 9375 and USB-6509. We use the USB-6509 with MOSFETs for voltage regulation and the NI 9375 to serve as a 24 V source, which keeps our system design simple because we do not need extra circuitry. We connected a current clamp to the NI 9205 analogue input module using its 16 differential inputs for measuring the current levels passing through the UUTs.
Our solution used the NI 9375 digital I/O module and the NI 9205 module within a cDAQ-9184 Ethernet chassis. We used the chassis’ modularity for a range of different outputs all transmitted to the computer through an Ethernet cable. Through the Ethernet connection, we created a test machine that we could remotely operate through the network. With the software drivers NI provided alongside our hardware, we focused on application functionality, instead of spending time programming low-level commands to communicate with the modules. We considered other hardware solutions but NI was a better choice for us due to the company’s customer service when troubleshooting technical issues.
In terms of the software, we use two programs to run the test, one using LabVIEW and the other using Visual Basic. We did this to keep the application backwards compatible with previously written programs for other projects, and to speed up the development process as we could easily reuse blocks of code.
We achieved the goals we set for this test application by using NI tools. We needed to ensure simplicity and fast development whilst keeping costs down. We have also created a repository of functions that we can easily export and use in future projects to further decrease development times. We no longer have to start from scratch thanks to the universal driver used across NI equipment, which means we save time and reduce costs. Even more importantly, with our new automated test system we can ensure every one of our circuit breakers keeps homes safe from electrical fires.
Last but not least, the project has just been assessed as outstanding by the KTP Grading Panel for its achievement in meeting KTP's Objectives of encouraging knowledge transfer activity between academia and industry.
Techna International Ltd
1 Metro Centre, Dwight Road
Watford WD18 9HG