A Current Limiting Fuse (CLF) is a fuse that has the ability to reduce the magnitude of fault current caused by a downstream fault. CLFs are typically used in high-fault-current areas to reduce fault duty on equipment. More recently, they are being used to reduce the severity of voltage sags. A Current Limiting Fuse meets the following three conditions: 1) interrupts all available overcurrents within its interrupting rating; 2) within its current-limiting range, limits the clearing time at rated voltage to an interval equal to, or less than, the first major or symmetrical current loop duration; and 3) limits peak let-through current to a value less than the available peak current.
Distribution transformers protected solely by an expulsion fuse have very low interrupting capacity, ranging between 600A and 3,500 Amps, and are subjected to higher risk of tank damage during an internal fault condition. These units also are subjected to large amounts of additional arcing and energy during an internal fault that may affect additional equipment on the distribution system. If a fault occurs that exceeds the expulsion fuse interrupting capability, the fault will continue to arc within the transformer until another protective device has time to react. This situation can increase the number of affected customers while extending the time required to restore service. The CLF high interrupting capabilities not only reduce the duration of arcing inside the transformer, but also isolate the fault to the transformer which minimizes the effects of the event.
- Major reduction in the depth of voltage sags and their duration compared to depth and sag when using an expulsion fuse
- An increase in safety for workers and the public because of greatly reduced damage at the fault location
- High cost compared to the cost of an expulsion fuse
- Incompatibility between fuse saving and CLF usage on lateral lines
- Susceptibility of CLFs to damage by transients
- An increased difficulty in coordinating with other protective devices
- ANSI C37.40 Standard for Current-Limiting Fuse Service Conditions.
- ANSI C37.41 Standard for Current-Limiting Fuse Design & Testing.
- ANSI C37.47 Standard for Current-Limiting Fuse Ratings & Specifications.
Padmount Transformers (Under Oil)
Current-limiting fuses can minimize the probability of catastrophic transformer tank failure on a distribution system due to high current faults. Sso due to safety concerns exposing the public to a catastrophic oil transformer tank failure, smaller distribution transformers in public spaces on residential, commercial and industrial feeders should have current limiting fuses. In Canada all single and three phase padmount transformers which are small enough to have an oil-immersed current limiting fuses must have them in conjunction with a bay-o-net weak link fuse. This is as per CSA-C227.4- Three-Phase, Pad-Mounted Distribution Transformers with Separable Insulated High-Voltage Connectors and CSA-C227.3 - Low-Profile, Single-Phase, Pad-Mounted Distribution Transformers with Separable Insulated High-Voltage Connectors.
Padmount Transformer and Switchgear (Loadbreak Elbow)
Full range current limiting fuses can be installed in specialized separable insulated connectors, and can be used to provide protection to downstream load from transformers. A full range fuse is a fuse capable of interrupting all currents from the maximum rated interrupting current down to the minimum rated continuous current that causes melting of fusible elements. The downside of this is that because the current limiting fuse is encapsulated in EPDM rubber, the continuous current rating is typically much smaller then a typical current limiting fuse or separable insulated connector. The seperable insulated connector may be normally rated for 200A continuous current where one with a current limiting fuse would be rated for only up to 45A.
Current limiting fuses will mitigate grass fires which can be started by either expulsion fuses in cutouts at overhead transformers or catastrophic tank failure. In addition where overhead lines are still prevalent in urban area's the space between the transformer tank and expulsion cutout fuse might be small to balconies, so having an extra layer of protection to the public is beneficial using current limiting fuses.
Capacitors and Harmonic filters utilize full range current limiting fuses. Capacitor fuses usually have a blown fuse indication to identify when they have operated to help locate failed capacitor banks.
Expulsion vs. Current Limiting Reaction to a Fault Current
The main difference in functionality between a CLF and an expulsion fuse is that the CLF limits the peak let-through current to a value less than the available peak current. In the below figure you see when an expulsion fuse will interrupt fault current on the waveform depicted on the left side. An expulsion fuse will interrupt when the waveform naturally arrives at the zero crossing, thus allowing for a lot energy let-though to be provided to the fault before this happens. On the right side is how the CLF interrupts fault current, it interrupt the fault current immediately within fractions of a cycle, and does not allow the fault to get to the peak, thus severely limiting energy let through to the fault.
How a Current Limiting Fuse Works
Current limiting fuses are typically constructed with an element surrounded by sand in an enclosed tube. The element passes current through the CLF during normal load conditions. This element acts like any conductor on the line. The element however is constructed with "weak points" which are key in extinguishing current during a fault condition.
Fuse Element with Narrow Weak Points
During the fault, the weak points vaporize and arcs develop between between the left over fuse elements. The arc’s melt surrounding sand, which creates fulgurite, a glass like substance.
The Elements Weak Points Have Vaporized and Fulgurite is Forming
Fulgarite is formed in nature by lightning, when lighting strikes silica and other conductive material, they are melted and fused together forming Fulgarite.
In the CLF, the fulgurite absorbs the heat from the arcs but also encloses them, so the arcs have to traverse more and more fulgurite, an extremely high resistance substance. The arc voltage increases dramatically, the resistance increases dramatically between elements, until current is drawn to zero. The energy let through is much less then available from the fault, the rest of the energy is dissipated in the fulgurite.
The Weak Points Completely Encapsulated in Fulgurite, Arc Extinguished
Types of Current Fuses
Full Range Current Limiting Fuse
A fuse capable of interrupting all currents provides both overload, and low and high current fault protection in a single fuse body. As a full-range fuse, it is capable of interrupting any continuous current between the minimum current that can cause melting of its elements and its rated maximum interrupting current (50,000 amps)Its ability to significantly limit energy let-through during a fault greatly reduces the likelihood of disruptive equipment failures. They are typically both indoor and outdoor versions. The fuse is hermetically sealed and thus discharges no gases during fuse operation like an expulsion would.
- Installed in drywell canisters for distribution transformer protection
- Clip mounted in live-front switchgear
- Externally mounted on overhead distribution systems
- Installed in rubber molded canister for deadfront/submersible applications
- Installed directly in oil
Backup Current Limiting Fuse
Backup current limiting fuses are used in series with another protective device, such as an expulsion fuse. This combination limits the high-fault current to a safe level, thus providing protection through the complete range.
Backup fuses typically are sold in version which are to be installed under oil within a transformer tank or in overhead outdoor versions installed on taps to poletop transformers, in series and upstream of the the expulsion cutout fuse.
The expulsion fuse is chosen to clear the low-magnitude currents such as those caused by faults that occur external to the transformer, high-impedance faults within the transformer, and in the case of load sensing links, transformer overloads. Such currents are generally below the minimum interrupting current of the current-limiting fuse. The back-up current-limiting fuse is selected as to clear all other faults up to maximum interrupting capability. In addition to interrupting the fault currents resulting from low-impedance faults within the transformer, the back-up current-limiting fuse serves the very important function of limiting the amount of energy that is let through to the source of the fault to a value below
the withstand capability of the transformer tank. .)
The current limiting fuse to the right, installed on the top of the transmission riser pole above the SMD expulsion fuse is manufactured by Hi-Tech, owned by ABB and can be installed on systems up to 138kV. Downstream of the the riser pole is a padmounted substation transformer which the CLF is protecting. These are special substations called Distribution Supply Centers which are smaller substation which contain only padmounted equipment and do not require to be fenced. This type of substation was created by Manitoba Hydro and do not contain breakers, thus require fuses to protect high value assets like the transformer. Downstream of the padmoutned transformer are padmounted distribution switchgear and padmount reclosers.