How to shield HMIs from overstress electrical surges

The main purpose of a human machine interface (HMI) is to provide an easy way to monitor and control industrial automation machineries. These components are usually integrated with programmable logic controllers (PLCs). And they are of significant importance in controlling the manufacturing processes of various routine use products.

HMI applications have to be very sturdy in order to operate in usually perilous environmental conditions. The main concern is that of EOS that is electrical overstress due to the fact that a lot of power surges that can be found in both industrial and manufacturing settings. This post will be focused on standard measures that enable shielding from hostile power surges and a typical component that is usually used for the protection of HMI applications.

If one takes a look at the data sheets of the semiconductor components that are integrated into HMI applications, one can see HBM and CDM electrostatic discharge evaluation grades. The human-body-model rating corresponds to the ability to resist ESD damage caused by human interference. And the charged-device model rating corresponds to the ability to resist ESD damage caused by the processes of mechanized production.

Neither of the two ratings fulfill the requirement of a rating to measure complete resilience of the system. As a result, a new standard was generated by the International Electrotechnical Commission for precisely this purpose. This system related rating corresponds to the overall protective ability against power surges of whole systems like HMIs. They are as follows:

• IEC 61000-4-2: electrostatic discharge immunity test.
• IEC 61000-4-4: electrical fast transient (EFT) immunity test.
• IEC 61000-4-5: surge immunity test.

The IEC 61000-4-2 assesses the system immunity with respect to higher level of power surges. The other two ratings correspond to additional levels of protection, which result in the production of a sturdy end-product.

TVS diodes provide safety level of IEC 61000-4-x for industrialized solutions which include HMIs. They should be integrated in extremely close proximity to any gateway interface which makes the HMI system prone to electric power surge damage. A TVS diode works by redirecting the high-surge current into the ground instead of letting it get to the HMI system. Apart from this, the design engineers can also make use of lower capacitance and easier routing package that is suited for various interfaces that exist in HMI solutions.