Sometimes product manufacturers change out the parts in a particular equipment model without changing the model number. Even though there is little to no change in function or feature set — the change is invisible to the customer — when regulations are involved these invisible changes can have severe consequences.
In the case of the Allen-Bradley Model 700-RTC relay, Allen Bradley added a component that changed the relay from merely a solid-state device to a digital device. “Solid state” refers to the way circuits are built: in solid materials. A solid-state relay, for example, performs the same function as an electromechanical relay, but it has no moving parts because it relies on transistors and thyristors to perform the switching instead of wire-wound coils and magnets.
A “digital” device refers to the method of signal processing. Digital is contrasted with analog; analog circuits may rely on continuous signals, like sine waves, to transmit information, where digital circuits rely on signal pulses that communicate discrete states: off and on, or 0 and 1. Digital circuits are built from analog components, and the analog components can be discrete or solid-state.
The key factor when using digital devices is the switching frequency. High-frequency currents radiate. Digital devices typically have on-board oscillators that produce high-frequency signals that are used to drive the digital logic circuits. The logic circuits are usually much more complex and customizable than those found in analog devices.
A vendor reported that they had previously classified the RTC relay as a solid-state device and tested it based on that information. With new knowledge that a complex programmable logic device was added sometime after the original testing, it would now be better classified as a digital device. The testing criteria change. Potential failure modes change:
Per NRC Part 21 notifications, UCI was informed that the Allen Bradley relays base model 700RTC contain a Complex Programmable Logic Device (CPLD) which was unpublished. This design change could not be noticed since the external appearance of the relay and the relay part number remained the same. Hence, UCI has qualified the subject relay as solid state relay whereas the presence of the CPLD device elects the item as a digital device which can be affected by EMI/RFI noises. At this time, UCI has no sufficient information to determine whether this design change would create a Substantial Safety Hazard as it relates to the plant applications for the subject relay.