400 & 500 Volt Diazed Fuse-Links to IEC 60269-3 – Type LD

400 & 500V Diazed Fuse-Links to IEC 60269-3 – Type LD Diazed Fuse | Screw Type Fuse | Bottle Type Fuse

Product Introduction

The Lawson Type LD is a comprehensive range of Diazed Fuse links for use in home, business, and building electrical installations at 400V and 500V a.c. As the definitive Screw Type Fuse for the Diazed D-type system, each Type LD Bottle Type Fuse is held securely in position by the screw cap of the Diazed fuse holder, which is screwed into the fuse base — giving a safe, positive, tool-free connection that has made the Diazed system one of the most trusted fuse formats in the world for over a century.

Available in both slow-acting and quick-acting variants across a wide range of body sizes — LD12, LD22, LD27, LD37, and LD51 — the Type LD Diazed Fuse covers current ratings from 2A to 200A, making it suitable for individual circuit protection right through to main distribution fusing in residential, commercial, and light industrial installations. The distinctive Bottle Type Fuse design — with a wider ceramic body tapering to a narrower end cap — is central to the inherent overrating protection of the Diazed system: the diameter of the end cap varies with current rating, and the gauge ring in the fuse base physically prevents a fuse of higher rating from being inserted.

In addition to the standard gG general purpose range, motor circuit protection and semiconductor protection variants of the Lawson Type LD Screw Type Fuse are also available — making the Type LD one of the most versatile Diazed fuse-link ranges available from a UK manufacturer.

Key Features

• Diazed Fuse — the standard D-type bottle-shaped screw-in fuse-link for 400/500V installations
• Screw Type Fuse — held in position by the screw cap of the Diazed fuse holder; no tools required
• Bottle Type Fuse — distinctive tapered ceramic body with varying end cap diameter for anti-overrating protection
• Slow-acting and quick-acting variants — slow-acting (LD series) and quick-acting (LDQ series) for different load characteristics
• Current ratings: 2A to 200A — five body sizes covering all standard circuit protection requirements
• Rated 400V and 500V a.c. — suitable for residential, commercial, and light industrial installations
• Five body sizes: LD12 (12mm dia.), LD22 (22mm dia.), LD27 (27mm dia.), LD37 (37mm dia.), LD51 (51mm dia.)
• Compliant with IEC 60269-3 — the international standard for fuses for use by unskilled persons in household and similar applications
• Motor circuit protection variants available
• Semiconductor protection variants available
• Compatible with Lawson Type LD Diazed Fuse Holder range (bases, screw caps, gauge pieces, and mounting plates)

Slow-Acting vs Quick-Acting Diazed Fuse — Choosing the Right Variant

Slow-Acting Diazed Fuse (LD series) The slow-acting (time-delay) Diazed Fuse is the standard gG general purpose variant — designed to tolerate short-duration current surges (such as motor starting inrush or transformer energisation) without nuisance operation, while still providing reliable protection against sustained overloads and short-circuit faults. The slow-acting Type LD Screw Type Fuse is the most widely specified variant for general circuit protection in residential, commercial, and light industrial applications.

Quick-Acting Diazed Fuse (LDQ series) The quick-acting variant (LDQ series) operates faster than the slow-acting type under overcurrent conditions and is used where rapid fault clearance is required — for example, in circuits supplying sensitive electronic equipment or semiconductor devices where a standard time-delay Bottle Type Fuse would allow too much energy through before operating. Quick-acting variants are available for LD12, LD22, LD27, and LD37 body sizes (2A–100A).

Applications

The Lawson Type LD Diazed Fuse, Screw Type Fuse, and Bottle Type Fuse range is specified in:

• Residential electrical installations — distribution board and consumer fusing in domestic properties with existing Diazed systems
• Commercial premises — general purpose circuit protection in shops, offices, and light commercial buildings
• Light industrial and workshop installations — sub-circuit and motor circuit fusing using slow-acting and motor protection Diazed variants
• Maintenance and replacement — like-for-like replacement of existing Diazed fuse-links in service
• European and international projects — widely used across Europe and internationally alongside UK installations
• Industrial control panels — motor circuit, transformer primary, and semiconductor protection using specialist Diazed variants
• Heritage and conversion projects — replacement fusing in listed and older buildings where the Diazed system is retained

Why Choose Lawson Type LD Diazed Fuse?

The Lawson Type LD Diazed fuse-link is used to prevent short circuit in home, business and building electrical installations, with the fuse held in place by the screw cap, which is screwed into the fuse base. Lawson Fuses The Bottle Type Fuse design’s inherent anti-overrating protection — where the varying end cap diameter physically prevents a higher-rated fuse from being installed — is a unique safety characteristic that has made the Diazed Screw Type Fuse system one of the most enduring and trusted fuse formats in the world.

With slow-acting and quick-acting variants across five body sizes, current ratings from 2A to 200A, and additional motor circuit and semiconductor protection variants available, the Lawson Type LD Diazed Fuse range is one of the most complete in the UK market. All products are manufactured under an ISO 9001 accredited quality management system with test facilities accredited to ISO/IEC 17025, and comply with IEC 60269-3.

Q1. What is a Diazed Fuse?

A Diazed Fuse is a D-type bottle-shaped ceramic HRC fuse-link used in the Diazed screw-cap fuse system. Introduced by Siemens in 1909 and standardised under IEC 60269-3, the Diazed Fuse is held in position by a screw cap carrier screwed into a fixed fuse base. The Lawson Type LD Diazed Fuse is available in 400V and 500V ratings from 2A to 200A across five body sizes, in both slow-acting (gG) and quick-acting variants, with motor circuit and semiconductor protection variants also available.

Q2. What is a Screw Type Fuse?

A Screw Type Fuse is a fuse-link designed to be secured in a fuse holder by a threaded screw cap mechanism. In the Diazed system, the Screw Type Fuse (the fuse-link itself) sits inside the screw cap carrier, which is then screwed into the fixed fuse base to make contact. The screw-in mechanism provides a secure, vibration-resistant connection and allows the fuse to be replaced tool-free by hand — simply by unscrewing the cap, removing the blown fuse, inserting a new one, and screwing the cap back in.

Q3. What is a Bottle Type Fuse?

A Bottle Type Fuse is the common informal name for the Diazed fuse-link, named after its distinctive bottle-shaped ceramic body — wider in the middle and tapering to a narrower end cap. The diameter of the end cap varies according to the current rating of the fuse: higher current ratings have larger end cap diameters, while lower ratings have smaller diameters. This dimensional variation is central to the Diazed system’s inherent anti-overrating safety mechanism — the gauge ring in the fuse base only accepts end caps up to a specific diameter, physically preventing a fuse of higher rating from being installed.

Q4. What sizes are available in the Lawson Type LD Diazed Fuse range?

The Lawson Type LD Diazed Fuse range covers five body sizes: LD12 (12mm diameter, 50mm length, 2A–25A), LD22 (22mm diameter, 50mm length, 2A–25A), LD27 (27mm diameter, 50mm length, 35A–63A), LD37 (37mm diameter, 63mm length, 80A–100A), and LD51 (51mm diameter, 66mm length, 125A–200A). The complete current rating range spans from 2A to 200A.

Q5. What is the difference between slow-acting and quick-acting Diazed Fuse variants?

The slow-acting Diazed Fuse (LD series) is a time-delay gG fuse that tolerates brief overcurrents — such as motor starting inrush currents — without operating, while still providing full overload and short-circuit protection. It is the standard choice for general circuit protection in residential and commercial installations. The quick-acting Diazed Fuse (LDQ series) operates faster under overcurrent conditions and is used where rapid fault clearance is required to protect sensitive equipment or semiconductor devices. Quick-acting variants are available for LD12, LD22, LD27, and LD37 sizes; the LD51 (125A–200A) is available in slow-acting only.

Q6. What voltage ratings are available for the Type LD Diazed Fuse?

The Lawson Type LD Screw Type Fuse and Bottle Type Fuse range is rated at 400V and 500V a.c. — covering both standard UK and European supply voltages for single-phase (230/240V) and three-phase (400/415V and 500V) electrical installations.

Q7. How does the Diazed Fuse’s anti-overrating protection work?

The anti-overrating protection of the Diazed Bottle Type Fuse system works through the physical relationship between the end cap diameter of the fuse-link and the gauge ring in the fuse base. Each current rating has a specific end cap diameter — lower ratings have narrower end caps, higher ratings have wider ones. The gauge ring fitted in the fuse base allows only end caps up to a maximum diameter to be inserted. This means it is physically impossible to fit a Screw Type Fuse of a higher current rating than the gauge ring permits, regardless of whether the installer is aware of the circuit’s rated current. This automatic, passive safety mechanism requires no monitoring or electrical intervention.

Q8. Are motor circuit protection and semiconductor Diazed Fuse variants available?

Yes. In addition to the standard slow-acting and quick-acting gG general purpose range, Lawson also offers motor circuit protection variants (gM) and semiconductor protection variants of the Type LD Diazed Fuse. These provide the specialised time-current characteristics required for direct-on-line motor starting and semiconductor device protection within the Diazed screw-cap fuse format. Contact the Lawson Fuses sales team for details of available ratings and variants.

Q9. What fuse holder is compatible with the Lawson Type LD Diazed Fuse?

The Lawson Type LD Diazed Fuse is designed for use with the Lawson Type LD Diazed Fuse Holder range — covering all sizes of fuse bases (DIII, DIV, DV), replacement screw caps, colour-coded gauge pieces, and surface mounting plates. The fuse base size must match the body size of the Diazed fuse-link being used, and the correct gauge piece must be installed in the base to enforce the appropriate current rating limit for the circuit.

Q10. How do I identify a blown Diazed Fuse?

A blown Diazed Fuse can often be identified visually — the ceramic body may show signs of blackening at the end caps, or the fuse indicator (if fitted) will show a visible change. The screw cap can be removed by hand and the fuse-link inspected. Always use appropriate electrical safety precautions when inspecting or replacing a Screw Type Fuse in a live installation, and follow the applicable wiring regulations and safety procedures for your installation type. Replace only with a Diazed fuse of the same type, size, and current rating.