Classifications can get messy. When taking my CID course, I was overwhelmed by the amount of knowledge to keep track of in the industry and related disciplines. Most of it was beyond retention for all but the most seasoned veterans. Individual rules or mandates can be difficult to recall. Still, best practices provide an excellent foundation for the motivation behind some tasks, whether reliability, performance, or other essential characteristics.
Still, there are certain cases where rules and regulations must be adhered to for the safety of operators. For Class 1 vs. Class 2 circuits, differing requirements between the classifications mean a misattributed feature could prove ruinous. To prevent this situation, carefully read and understand the similarities and differences between these two standards.
An Introduction to Class 1 Circuits
Class 1 circuits exist as the part of the wiring system that interfaces the load-side of power-limiting equipment and any integrated equipment. These circuits are further divided into two subgroups: power-limited and remote-control and signaling circuits. The immediate distinguishing factor is the source properties. Power-limited must have a source of 30V and an apparent power less than 1000VA, respectively, while a remote control and signaling must have a source of 600V or less.
Class 1 represents a heightened sense of damage relative to their environment: failures to the circuit, cables, or box could cause fire or some additional life hazards to nearby operators. Safety-control equipment notably does not include items like household control systems, like home heating and air. For the remote control systems that meet the qualifications of Class 1, these circuits must be confined within an enclosure to prevent any damage to the surrounding environment and protect the circuit itself.
There’s far more to the considerations of a Class 1 circuit, however:
- Power supplies. With the noted exceptions of transformers, power supplies need overcurrent protection of no more than 167% of the apparent power divided by the rated voltage. Non-transformer power supplies will be limited to less than 2500 apparent power and 10,000 maximum apparent power (measured over one minute with overcurrent protection bypassed, load notwithstanding).
- Overcurrent. Overcurrent protection can be placed where the conductor interfaces with the supply or on the input side of the source so long as the product of the current-carrying capacity of the conductor and the voltage gain are within acceptable limits. With the exception of two-wire, all outputs will not be accepted as overcurrent protected lest they satisfy the above.
- Coexistence. Class 1 circuits, regardless of the current method (direct or alternating), may be placed within the same enclosure in some cases on the condition that any conductor is insulated for the maximum voltage present within the space.
Class 2 Circuits (Bonus Class 3 Circuits Included)
Class 2 circuits sit between the load side of the appropriate class source and any connecting equipment. These circuits and Class 3 focus on fire prevention as their foremost safety goal, with Class 3 having some additional protection against shocks over Class 2. Like the remote control and signaling Class 1 circuits, the voltage of the source cannot exceed 600V.
Class 2 and Class 3 circuits, much like differing Class 1 circuits, can be installed together, provided there is a barrier between the two circuits. Any class circuits can be installed within the same enclosure following this stipulation. Further, Class 2 and 3 circuits can be placed with enclosures with single fitting openings (such as a tee) alongside Class 1 circuits (so long as there exists a nonconductive material separating the two circuits). Enclosures that do not adhere to the above designs must provide, at minimum, a 50 mm air gap between Class 2 and 3 circuit conductors and those of Class 1.
Class 2 and 3 circuits can also be placed together with a common jacketed cable in the following applications:
- Power-limited fire alarm system
- Non-conductive and conductive fiber optic cables
- Circuits used for communication
- Community antenna television and radio distribution
- Low-power broadband communication
- Audio system circuits
Class 1 vs. Class 2 Circuits? Your Contract Manufacturer Can Sort It Out.
Class 1 vs. Class 2 circuits are not a conflict in design but clear-cut methods to ensure that technical specifications are met for extremely high-power circuitry. There is significant overlap (and important distinctions) between the two classes. Designers and manufacturers must remember that these installations’ primary purpose is to provide a safe enclosure for operators.
Still need some assistance working through the specifics of the respective classes? Your contact manufacturer has everything sorted out from beginning to end to ensure that every design element meets specifications. Here at VSE, engineers are committed to building electronics for our customers. Along with our valued manufacturing partners, we will ensure your board is looked after as if it was our own, from design through manufacturing.
