On a 5–38 kV distribution loop, load break switches (LBS) are the workhorses used to break the ring into dead segments after upstream protection has cleared the fault current, since an LBS is rated to open normal load current but not to interrupt the short-circuit current itself [S3]. The standard field move is to split the loop at its LBS access points until the faulted cable section is bracketed, then open the two LBS units flanking the fault, park the elbows on stand-offs, and prove dead before crews touch the conductor [S4].
The reason LBS, not isolators, sit at every loop-access point is that they can make and break the feeder load (typically 400–1250 A on medium-voltage systems) under SF6, vacuum, or air arc-control, while a non-load-break isolator is an "offload device" that can only be operated when current is zero [S3][S1]. For a deeper look at the switch category itself, see the load break switch reference page.
What "Loop Isolation" Actually Means in the Field
Loop isolation is the practice of breaking a closed ring-main feeder into two open ends around a faulted section so the bad cable can be lifted, repaired, or replaced while the rest of the customers stay energised through the alternate feed [S3]. Load break switches placed at every transformer or sectionalizing junction make this possible because they can be operated under load without causing the kind of arc-flash damage a plain disconnect would suffer [S1].
Per field procedure references, the first mechanical step after the upstream protective device has opened is to "lift the load-break elbow for the first section of cable at the first transformer on the isolated side of the loop and place it on a stand-off," then work back toward the source, parking each elbow clear of its bushing until the faulted span is bracketed between two open LBS positions [S4]. This is why utilities spec dead-break or load-break separable connectors at every sectionalizing point, not at just the ends.
Why LBS, Not Isolators or Breakers, Sit at Every Sectionalizing Point
A load break switch can "make, carry and break currents under normal circuit conditions, including specified overload conditions, and also carry for a specified time currents under abnormal circuit conditions such as short-circuits," which is the IEC 62271-103 definition and the engineering reason it is the right device for loop sectionalizing [S3]. An isolator (non-load-break switch) is exactly the opposite, it is "an offload device" that can only be operated when the current running through is zero, and forcing it open under load "can cause an arc flash, which could in turn result in a fire outbreak, damage to properties, or even injure you" [S1].
A circuit breaker, by contrast, is designed to automatically clear fault currents and is rated to interrupt 10–40 kA or more, far more capability than an LBS, but it is also far more expensive per unit, so utilities do not install a breaker at every transformer; they install LBS where they need switching points and rely on the substation breaker plus fuses for fault clearing [S2][S3]. A typical 15 kV or 33 kV LBS sits in the same voltage class as the feeder and handles the normal 400–1250 A load band, with a short-time withstand rating for the rare moment a fault is being cleared upstream [S2].
Step-by-Step Procedure: Bracketing a Fault on a Pad-Mounted Loop
The accepted field sequence is to first identify which LBS the upstream protection opened, then walk the loop in both directions, parking load-break elbows on stand-offs at each sectionalizing transformer until the faulted cable is bracketed by two open switches [S4]. Crews use a thump or low-voltage tester to prove dead after each opening, because the goal is to create a visible open point on both sides of the suspected fault before any splicer enters the cable.
Key decision points: the LBS can be operated manually with a hotstick or remotely when remote control is fitted, and remote operation is preferred in storm or emergency conditions because "operators can remove power from a safe distance" [S5]. The LBS does not need special tools to operate, which is why it is specified for routine sectionalizing rather than a tool-required isolator that would slow restoration [S5].
Limits You Hit When the Fault Current Is Still Present
The hard rule is that an LBS will not interrupt fault current, so "attempting to open a load break switch under a short-circuit condition could lead to severe arcing, equipment damage, and pose a significant safety risk" [S2]. That is why the upstream protective device (substation breaker or line fuse) must open first, and the loop-sectionalizing LBS is only operated after the fault has been cleared and the feeder is carrying load again through the alternate path.
The LBS carries the maximum fault current only "till the interrupting device clears the fault," meaning its short-time current rating (typically 1–3 seconds at values set by the system study) is the bridge between fault onset and breaker trip, not a substitute for it [S1]. For pad-mounted gear with SF6 or vacuum interrupters, the arc-control medium is what lets the LBS open 400–1250 A of normal load cleanly; the same contacts will not survive a 10 kA arc, which is why fuses or breakers ride upstream of every LBS loop [S3].
Comparison: LBS vs Isolator vs Breaker for Loop Sectionalizing
On four decision criteria, the three devices line up as follows. Function: LBS opens and closes under normal load, isolator only opens at zero current, breaker opens automatically on overload and fault [S2]. Fault-current interruption: LBS negligible or none, isolator none, breaker high (10–40 kA typical on MV systems) [S2][S3]. Operating mechanism: LBS manual or remote with quick-make/quick-break spring, isolator slow manual with tool, breaker stored-energy trip [S2][S1].
Cost and placement: LBS sits at every sectionalizing point because it is cheaper than a breaker but can still be operated under load, while isolators only appear where a visible break is needed and zero current is guaranteed [S1][S3]. In a typical 5–38 kV loop, the LBS is therefore the device that does the bracketing work, the breaker does the fault-clearing work, and the isolator only shows up as a visible disconnect downstream of an opened LBS.
What Crews Verify Before Restoring the Loop
Before the two bracketing LBS units are closed to re-energize the repaired section, the standard checks are: visible open on both sides, elbows parked correctly, test ground sets applied, and a phasing check across the open because the alternate feed may be on a slightly different phase angle after reconfiguration through the other path [S4][S3]. LBS units with motor operators can be closed remotely once the crew is clear, which avoids a second walk-out and is the practical reason modern pad-mounted switchgear ships with remote-ready actuators [S5].
A trackable signal for the next maintenance window: inspect every load-break elbow interface for partial-discharge noise and check the SF6 gas pressure (or vacuum integrity) on each LBS, because a gassed-up or wet elbow is what turns a clean sectionalizing job into a stuck switch on the next outage. For related field-service detail on connector wear and similar contact-interface decisions, the high-flex shielded robot cable spec guide covers parallel rules on bend radius, shield termination, and flex life that apply to any elbowed medium-voltage cable. For sizing the upstream protection that has to clear the fault before the LBS is operated, the 10 GPM at 3000 PSI motor kW sizing walkthrough is a useful parallel on matching device rating to the load band.
For the relevant spec sheets and selection criteria, see steel section, and loop calibrator.