"We provide a lot of after-sales support for our machines. One of our headaches is control circuit (24 V) protection, particularly identifying and troubleshooting blown circuits. The other part of this that's becoming a pain is stocking and supplying lots of types of fuses and circuit breakers. We know we can justify the extra costs for both our customers and ourselves if we migrate to electronic circuit protection. How do we make that case to our customers?"
—From June '13 Control Design
Here's the Case to Make
It sounds like you're experiencing all the common pitfalls of traditional 24 Vdc circuit protection from breakers or fuses that have been adapted to a control-level-sensitive world. Justification to make the leap to electronic circuit protection is easier than you might think. Consider the downtime costs associated with a tripped circuit. First, there's recognition of a problem. Is it in a cabinet? Which circuit? Where on the circuit? Will the breaker reset, or do I replace the fuse only to have the same problem again when the resistance in the wires reaches a temperature or current draw with enough impact to fault again? How many people need to be involved to diagnose and repair the problem? All this, and you're still where you were in the beginning.
Sell this to your customer: Mr. Customer, our newly designed control cabinet uses state-of-the-art protection for 24 Vdc circuits. This means better protection for the devices on the circuit, less heat on the cables before a fault is detected, a notification of a fault or warning of potential overcurrent before a fault happens, and fault detection in milliseconds. Visual indication of the affected channel is possible through an HMI or other device, and you can test and reset without opening the cabinet and disturbing the wiring inside.
Sell this to management: Fewer service calls for "ghost" short circuits, better monitoring of circuit load, reduced inventory, reduced wiring, fewer opportunities for the user to open the cabinet and disturb the wiring.
As for the hard costs to the initial build, you should quickly realize savings in assembly, procurement, and inventory costs, which make the switch to electronic protection the easy and right choice. Offer protection and detection not reaction and inspection.
Complexity Brings Vulnerability
The use of analog and digital I/O signals have multiplied greatly, as discrete systems and processes become more integrated and compact. Integrating separate functions and controls, and reducing overall size of the control system is a very desirable goal. However, by doing this, the control device and process becomes very vulnerable to damage from voltage transients (surges). Because the modernized devices perform more functions, the impact is greater overall system interruption. To ensure system reliability, proper implementation of surge-protection devices (SPD) will improve signal integrity greatly, protecting both devices in the field and in the control panel.
Best practice for mission-critical applications is to protect all wiring that leaves a controlled environment greater than three meters. A controlled environment should be considered a safe place for controls. A controlled environment would be an RTU control panel enclosure or perhaps a room such as a SCADA room. The only way to assure that an RTU or SCADA structure remains a controlled environment is to install appropriate SPDs on all galvanic wiring that enters or leaves that space. If not implemented, these wires are potential points of entry for damaging surge events. If this happens, then technically the space can't be considered as a whole, controlled environment.
Fusing and circuit breakers don't protect against voltage surges, only over-currents that originate on the load end of a circuit. SPDs protect against over-voltage transients. SPDs have extremely fast response times, normally around 25 ns. This is critical as surges are extremely fast. A fuse or circuit breaker does not come close to the performance or speed of an SPD.
The job of an SPD is to be invisible to the circuit, and at the same time, be a voltage-activated switch that diverts the damaging voltage surge to ground, away from the circuit that is to be protected.
product marketing lead specialist,