Application Analysis of Load Switch and Current-Limiting Fuse Combination Apparatus

06/27/2025

As a front - line technician in loop - network power supply and prefabricated substation operation and maintenance, I deeply understand equipment iteration driven by high - voltage urban expansion. Per the National Power Supply and Consumption Regulations, for equipment over 250kW or 160kVA transmission capacity, 10(6)kV high - voltage power supply and 220/380V step - down form a necessary pattern, making loop - network units and prefabricated substations key in distribution networks.

I. Equipment Structure and Protection Scheme Selection
(I) Equipment Composition

Loop - network units I handle usually have 2 loop cable intervals and 1 transformer circuit interval. Prefabricated substations integrate high - voltage switches, transformers, and low - voltage devices into compact, prefabricated sets for indoor/outdoor use. The core is high - voltage switches’ protection against transformer faults (e.g., short circuits).

(II) Protection Scheme Comparison

In practice, I tested two protection methods: circuit breaker and load switch + current - limiting fuse. The latter is superior —— simple, cost - effective, and more effective for transformers. Short - circuit tests show transformers need short - circuit clearing within 20ms to avoid tank explosions; current - limiting fuses do it in 10ms, while circuit breakers take ~60ms (relay + operation + arcing time), so I prefer the fuse scheme.

II. Necessity of Load Switch + Current - Limiting Fuse
(I) Application Advantages

Most domestic and foreign loop - network/prefabricated substation projects I participated in use load switch + current - limiting fuses. They feature simple structure, low cost, and good protection for transformers. Short - circuit tests (verified on - site) show fuses clear faults in 10ms (vs. circuit breakers’ ~60ms), critical for preventing tank explosions.

(II) Cooperative Logic

Fuses may cause unbalanced phase operation if single - phase fusing occurs. Thus, load switches must cooperate: fuse strikers trigger load switch tripping for three - phase breaking —— a verified, indispensable coordination.

III. Cooperation Key Points of Load Switch and Fuse

As a front - line worker, I know their cooperation is vital. The IEC420 standard defines rules, dividing current into 4 regions (my debugging basis):

$(I) Region I (I < Iak)$

I ak

(combined appliance rated current) is less than fuse rated current

I a.nT

(due to installation temperature/heat loss). Load switches break rated current and extinguish three - phase arcs —— my daily inspection focus.

(II) Region II (Ia.nT< I < 3Ia.nT)

In overload, fuses bear over - current first. At ~

2I a.nT

, melts act (but not arc - extinguishing), strikers trigger load switches for three - phase breaking. I test this time - difference logic to avoid protection failure.

(III) Region III (Transfer Current ITC, ~3Ia.nT Start)

Fuses can extinguish arcs after action. One three - phase fuse acts first, triggering strikers; load switches extinguish other two - phase currents. The key is transfer current (load switch’s max breaking current at specific power factor,

5I a.nT

15I a.nT

), checked during selection/verification.

(IV) Region IV (Current - Limiting Range)

For extreme faults, fuses act in the first half - wave to limit fault current peaks; load switches act but don’t break current. I verify this logic in drills for proper operation.

IV. Transfer & Hand - over Current Requirements

These parameters ensure equipment safety, key for my on - site debugging:

(I) Transfer Current

It’s the critical value for function transfer between fuses and load switches. Below it, fuses break one phase, load switches handle the rest. Striker - equipped load switches need transfer current tests (usually > rated current) —— a challenge for old equipment, verified per IEC420.

(II) Hand - over Current

It’s the current fully broken by load switches (no fuse participation). For load switches with both strikers and releases, hand - over current tests are needed. If hand - over current > transfer current, transfer tests may be exempt. Release operation reduces fuse loss but increases vacuum load switch costs (adding relays/releases) —— trade - offs made per project budgets/conditions.

V. Transformer Protection Suggestions

For load switch + fuse transformer protection, key verifications include:

Striker Tripping: Check matching of actual and rated transfer currents for safe breaking.
Over - current Release: Verify actual and rated hand - over currents for reliable operation.

These tasks are mandatory for new projects/old equipment transformations. As a front - line worker, I ensure stable power supply and safe fault handling for downstream users.

Topics

Load switch Applications and Trends

Hey! I'm Oliver Watts, an electrical engineer in Inspection and Testing. With years of hands - on experience, I ensure electrical systems meet top safety and performance standards. Using advanced gear, I conduct diverse tests, easily spotting issues in both large - scale industrial and small - scale commercial setups. I love teaming up, sharing knowledge, and keeping up with industry regs. Also, I'm skilled at data analysis with software. If you're into electrical inspection or just want to chat engineering, reach out. Let's connect and explore!