Secure circuit protection

Every systems manufacturer—from machine builders to process engineers—must consider circuit protection. Whether protecting against overloads or short circuits, fuses protect equipment and wire conductors from failure. Fuse elements have not changed in years because of their dependability. Operators also can reduce wiring time by selecting fuse terminal blocks with push-in connection technology (Figure 1). 

Fuses work for field use

Fuses were one of the first components developed in electrical engineering. In 1880, Thomas Edison culled the idea to protect an electrical circuit with a predetermined breaking point and filed a patent on this concept. In the event of an overload, the electrical current is concentrated on a defined point by intentionally narrowing the conductor cross-section in the circuit. This basic principle is still implemented today in most fuses.

The purpose of a fuse is to protect equipment and conductors from failure. In the event of a short circuit in device circuitry, for example, a brief overcurrent causes the fuse element to trip and the circuit to open before the device is damaged. In this case, it is vital that the fuse element and the corresponding fuse carrier-a special terminal block-fit the application.

In machine building and systems manufacturing, 5×20 mm and 6.3×32 mm cartridge-type fuses are commonly used. They are becoming more prevalent when reliable and accurate circuit protection is required. Due to a wide range of fuse types—fast blow or slow blow, sand-filled, in glass or ceramic housing—cartridge fuses are available in many versions. The cartridge design of fuse links is a global standard and has remained consistent for decades since its first inclusion in DIN 41571, the international standard for miniature cartridge fuses. 

Fuse terminal blocks, push-in connections

Driven by key trends in cost efficiency and miniaturization, a variety of fuse terminal blocks have been developed. To support market demand, 4-mm² multi-level fuse blocks are available for 5×20 mm cartridge fuses and 6-mm² fuse blocks for 6.3×32 mm cartridge fuses. The series includes disconnect terminal blocks with housings to match the fuse block. This allows field application design flexibility (Figure 2).

Fast and easy push-in connection technology is gaining acceptance in many markets and industries. Seven advantages compared to traditional connection technologies include:

1. Pressure-spring principle operation. With a leg spring, the conductor is plugged into the terminal without the need to first open the terminal point. The leg spring consists of high-strength chromium-nickel steel. The conductive metals made of copper-alloy are protected by lead-free galvanic tin plating. Using similar metals reduces the impact of thermal expansion for copper conductors. The leg spring, based on a sophisticated design, pushes the conductor against the current bar, enabling the conductor to be inserted with 50% less insertion force compared to other spring technology products. This reduces wiring time and delivers reliable connections and time savings for the builder and end user.

2. Compact housing design. In space-sensitive applications, new multi-level fuse terminal blocks save time and space. In conventional sensor/actuator wiring, the power supply and the signal paths are wired separately on adjacent feed-through terminal blocks. These fuse blocks also accommodate wiring for the ground potential of the conductor shield. Greater distances to the control cabinet require larger conductor cross-sections.

In the past, this application would require five terminal blocks. With the newer fuse terminal blocks, this application requires one pair of terminal blocks: one multi-level fuse terminal block and one disconnect terminal block. Taking advantage of a high-density housing width of 6.2 mm and reducing the number of terminal blocks required from five to two can save 18.6 mm of space per assembly (66% when compared to the standard 31 mm).

3. Terminal block accessories. Professional marking to clearly identify each terminal point and conductor location, potential distribution with reliable jumpers, and dual bridge shafts are available (Figure 3).

4. Simple intuitive fuse terminal block assemblies. Field operation and replacement of a blown fuse element should be fast to minimize system downtime. Automotive flat-type C fuses are used in automotive and machine building manufacturing. To meet this growing demand, a fuse terminal block for compact type C fuses according to ISO 8820-3 and DIN 72581-3 is available.

Flat-type C fuses are color-coded to denote current strength. These fuses are available for low voltages up to 48 V, but the fuse carrier can be used for other fuses, including higher voltages up to 250 Vac. The standardized flat-type fuse socket can hold thermal circuit breakers (TCP) that convert a fuse terminal block into a miniature circuit breaker.

In addition to the high nominal voltage, thermal circuit breakers have other benefits. These automatic devices can be switched on/off, and current paths can be enabled using an operating element. In contrast, fuses cannot be repaired and therefore must be replaced after tripping.

5. Less terminal width. Terminal width is significantly less conventional miniature circuit breakers. This saves space in the control cabinet. Flat-type C fuse terminal blocks are available with a cross-section of 6 mm², supporting nominal current of up to 30 A-a performance operating range common for many electrical devices (Figure 4).

6. Standard testing. IEC 60947-7-3 terminal block tests are described in the fuse terminal blocks standard. Specific tests are performed on fuse terminal blocks, including testing for environmental, shock and vibration performance.

7. Testing power dissipation. Due to the nature of fuses, heat is generated during nominal operation. However, temperature limits of plastics and metal parts that an operator may come in contact with must not be exceeded. To determine the limits, fuse terminal blocks are equipped with thermocouples, which monitor the temperature in one arrangement and in a composite arrangement. The value for plastic parts must be below the relative temperature index (RTI) value, which is 130°C for Polyamide 6.6. The temperature for metal parts must not exceed 85°C. As a result, maximum nominal current and maximum power dissipation for the fuse must be strictly followed.

Automotive fuses, according to ISO 8820-3, have derating curves that must be used in accordance with the fuse manufacturer. In general, ISO 8820-2 recommends a maximum load current of 70% and DIN 72587-3 a maximum load current of 80%.

Alan Sappe’ is product marketing manager, industrial cabinet connectivity business unit, Phoenix Contact USA; Cory Myer is product specialist for fast connections, industrial cabinet connectivity, Phoenix Contact USA; and Moritz Krink is product manager terminal blocks, industrial cabinet connectivity, Phoenix Contact GmbH & Co. KG; Edited by Mark T. Hoske, content manager, Control Engineering, CFE Media, mhoske@cfemedia.com.

KEYWORDS: Power protection, industrial fuses

• Short-circuit protection is available in push-in fuse terminal blocks.
• Push-in connection technology is gaining acceptance.
• Advantages compared to traditional connection technologies.

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