Current limiting fuses

ANSI C.37.47/1992 Specifications for distribution fuse disconnecting switches, fuse supports and current limiting fuses Motors and generators ratings, costruction. testing and performance... 

Potential/control transformers must be provided with current limiting fuses at both ends. 

High voltage fuses - Current limiting fuses 9385-1 to 5 BS EN 60282-1/1996... 

Fuses should not be used to protect secondary voltage feeders. The time current characteristics of fuses above 100 A will not coordinate with the groimd fault pickup currents and time delays of the main overcurrent protection (circuit breaker or fused disconnect switch) ground fault protection. A main load break disconnect switch can be equipped with current-limiting fuses to reduce the available short-circuit current from the utility and should have a three-phase voltage relay for single-phase protection. 

The real world test parameters discussed above have been incorporated into the fire hazard assessment model used by UL (Webber 1983) to determine the compliance of transformer fluids with section 450.23 of the National Electrical Code. In order to be compliant with the code, transformers insulated with less-flammable liquids are permitted to be installed without a vault in Type I and Type II buildings of approved noncombustible materials in areas in which no combustible materials are stored, provided there is a liquid confinement area, the liquid has a fire point of not less than 300 C and the installation complies with all restrictions provided for in the listing of the liquid. UL has identified the need for pressure relief devices and current limiting fusing to limit the effect of possible high current arcing faults. 

Current limiting fuses may be used in place of PTC devices when a sustained discharge is not preferred. Fusing of this type may utilize slow blow-time fuses or fast-acting fuses with little current-time latency. Time delay and specially fast-acting fuses are external to a cell however, fusible links may be installed in the cell. 

Fault condition particularly when the LT distribution is fed through a large transformer and the outgoing feeders tre protected by a current limiting device, HRC fuses or breakers. In the event of a fault, on a large... 

Examples are HRC fuses (both LT and HT) and MCCBs and MCBs (LT only), which are available with current limiting features and are in extensive use. The tripping time of these devices is extremely low and much less than one half of a cycle of the current wave. They therefore do not allow the fatilt ctirrent to rise to its prospective peak. The protected devices and components can thus be selected based on the let-out energy of such devices on fault, which is extremely low, than the fault level of the system. If... 

Output fuse failure Test current limitation, remove overloading or short circuit, set control device... 

The precise nature of bifurcation at IV4 = N r (likewise, for other Ni, i = 1,2,3). It is expected that at JV4 = Nf the lower and middle limiting current branches fuse and annihilate each other, leaving an inflection point at the voltage-current curve for N4 < N". ... 

Service entrance equipment should be installed in an indoor electrical room whenever possible. The equipment should include one to six main service entrance power circuit breakers or load break disconnect switches and feeder circuit breakers for loads within the plant. Circuit breakers and disconnect switches for 480/277 V systems that are rated at 1000 A or more must include groimd fault protection. Feeder circuit breakers will typically be molded case. Integrally fused circuit breakers are available with very high short circuit lAC ratings for both power and molded case circuit breakers. Fuses and current limiters for integrally fused circuit breakers must be stored within the electrical room. 

However if the prospective current is not at its maximum then several cycles of current may occur before sufficient heat is created to melt the fuse. In this situation the fuse is said to be non-current limiting. This applies to times beyond about 0.01 sec on the fuse cmve of Figure 8.2. As the prospective current is reduced the non-current limiting time, or operating time, increases considerably. A particular design of fuse may take several horns to operate if the prospective current is only a small amount above the asymptotic value of the fuse. Four hours is used by manufacturers as a reference value. It can be seen therefore that times less than 0.003 sec are important when high currents occurs. 

The manufacturers use this procedure to extrapolate their cmves below 0.01 sec and C is therefore a theoretical time. Virtual time is related to the prospective current by definition and so the manufacturer will quote the maximum prospective current that can be used in conjunction with his curves. At this point the engineer is encouraged to consult the manufacturers for advice on the selection of fuses for current-limiting duty. 

Fuses. Several types of fuses are in use standard one-time, time-delay (or dual-element), and current-limiting. The standard one-time fuse has no time-delay feature. The time-delay dual-element fuses provide time delay where a heavy overload current might exist for a short time (normal motor starting).This type of fuse will not open the circuit during this brief time however, if the overload continues, the fuse wiU cause the circuit to open. In a short-circuit condition, this type of fuse wiU open the circuit in an extremely short time. 

Current-limiting-type fuses are used where extremely high short-circuit currents are available. They react to open the circuit in a fraction of a cycle, thereby limiting the actual current that can flow. 

Power Fuses (Over 600 V). The NEMA E rating for power fuses requires that all fuses must carry their current rating continuously. Two types of power fuses are in common use current-limiting and expulsion type. 

Current-Limiting Type. These power fuses are designed in such a way that the melting introduces high arc resistance into the circuit during the first half-cycle s peak current, thus restricting short-circuit current. 

High currents (short circuit) Limitation of short circuit current by fuses or circuit breakers Short circuit safe installation of leads... 

Short Circuit Protection Normally, current limits are incorporated into the protection circuits. As a backup, a PTC device or fuse is placed in series with the battery pack. It is advisable to place the PTC between the pack assembly and the output of the battery. By placing it at this point, the PTC wUl not interfere with the operation of the upper or lower voltage detection of the electronic control circuit. 

Overheating Limit current drain Fusing, thermal cutoff, PTC devices Design battery properly Do not incinerate... 

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