These devices have been used since the late 19th century and continue to be applied today. However, circuit protection continues to evolve with ever-changing technology. Today, there are technologies that use intricate communication and control strategies and can report which type of overload or fault opened a breaker, provide insights on power quality, measure harmonics, alarm certain events like ground fault, and more.
The most basic levels of circuit protection include fuses and thermal magnetic-type circuit breakers. Fuses contain a fusible element, which responds to the heat generated by the passage of current through it with a typical operating curve. A typical thermal magnetic circuit breaker includes a long-time trip operation region as well as an instantaneous zone. Some are adjustable in the instantaneous region, but these, along with fuses, are not inherently smart devices and have no built-in intelligence.
They offer basic wire and equipment protection. They are designed to "break the circuit" when a fault occurs beyond their operating range. A distribution system should be designed when the OCPD isolates a fault close to the event without affecting unnecessary equipment upstream. This is referred to as selective coordination. With a standard fuse, or thermal magnetic device, you have basic circuit protection, but due to limited flexibility, they only offer basic protection from significant arc flash dangers. A thoughtful design assures the downstream feeder breaker has enough time to "clear" before the fault condition pushes the upstream breaker into its trip curve.
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In Figure 3, the diagram on the left shows a system lacking selective coordination. The upstream and downstream devices highlighted opened, since the OCPD closest to the fault did not trip first, thus all the red devices and their associated downstream loads would see an unnecessary power loss. Again, in Figure 3, the diagram on the right indicates how a correctly coordinated system would isolate the fault condition as close as possible and leave the rest of the system up and running as usual.
A means to produce a more reliable and coordinated system is to add intelligence to a circuit breaker in the form of integral trip units and protective relays. Another type of circuit breaker is an electronic trip-adjustable circuit breaker. This breaker has a long time-operating region, a long-time delay, a short time pickup, a short time delay, and finally, an instantaneous pickup.
These parameters are adjustable over a given range. This adjustability makes the electronic-trip circuit breaker very flexible when coordinating with other devices. However, these devices are still not "smart" devices. The settings are initially set, but they are not communicating with other devices to provide optimum protection.
Electronic breakers allow a design to be better coordinated, but they still tend to drive the circuit breaker sizes higher the further upstream you go to minimize the overlap in the trip regions. A design engineer must use experience and judgment to optimize the inherent trade-off for reliability and safety.
Overload or Short Circuit Protection? How to Protect Your Design Against Both Dangers | E-T-A
The engineer must be careful; if he or she designs an electrical system based solely on safety by minimizing arc flash, it will be difficult to coordinate all devices. The system could be plagued with nuisance tripping, and costly unplanned downtime would be imminent. Likewise, designing a system singularly focused on uptime would place people at risk as well as the plant equipment.
ZSI consists of wiring two circuit breaker trip units together so a fault is cleared by the breaker closest to the fault in the minimum amount of time possible. The primary goal is to switch off the fault current within the shortest time possible while impacting the least amount of connected equipment. ZSI is not new a technology, but tends to be more expensive. Manufacturers have different ways of accomplishing the same principle, so it is important to understand the nuances.
However, the NEC added a requirement to provide arc-energy reduction Article Additional circuit-protection strategies include using protective relays in the OCPD. Protective relays and devices can be applied to a system to help protect the circuits from conditions, such as reverse-power flow, single phasing, or transients and surges.
Directional power or reverse-power relays monitor the direction of current and have the ability to respond by disconnecting the circuit. Differential relays measure the difference between two values of current and respond accordingly if it senses an error. Electromechanical devices may not have the reserve capacity to open safely when a second or third fault occurs. When a fuse opens it is replaced with a new fuse, so the protection level is not degraded by previous faults.
Fuses typically are the most cost-effective means of providing overcurrent protection.
Electrosensitive protective devices for safe machines — Part 1
This is especially true where high fault currents exist or where small components such as Control Transformers or DC power supplies need protection. Devices with low interrupting ratings are often rendered obsolete by service upgrades or increases in available fault current. Updated NEC and UL standards are causing the need for potentially expensive system upgrades to non-fused systems. Fuses can be easily coordinated to provide selectivity under both overload and short-circuit conditions. Fuses do not require periodic recalibration as do some electromechanical overcurrent protective devices.
As a fuse ages, the speed of response will not slow down or change. Here are some commonly used fuse terms:. To read more articles about circuit protection, click here. Why Use a Fuse? Safety Overcurrent protective devices that have tripped are often reset without first investigating the cause of the fault. Published in: Engineering. Full Name Comment goes here. Are you sure you want to Yes No.
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No Downloads. Views Total views. Actions Shares. Embeds 0 No embeds. No notes for slide. Electrical protective equipment 1. Topic : Electrical Protective Equipment 2. If current in the circuit exceeds more than the rated current, then the heat generated also exceeds and which causes the meltdown of fuse wire. Unlike the thermal switch which automatically resets itself when the temperature drops.
For example, a glass tube fuse rated at 32 volts would not reliably interrupt current from a voltage source of or V. If a 32 V fuse attempts to interrupt the or V source, an arc may result. Rated voltage remains same for any one fuse, even when similar fuses are connected in series.