Fibres described as 250 µm and 900 µm differ in construction and in the amount of material around the glass. This affects splice preparation, the entry space required inside a protector and series selection. This guide explains what to measure and how to avoid common mistakes.
Key point: 250 µm and 900 µm do not describe the glass diameter. A common telecommunications fibre has 125 µm glass cladding; the larger values refer to protective layers. Select the protector for the actual construction, prepared length and tray.
How is an optical fibre constructed?
The centre consists of a core and glass cladding. In common telecommunications fibres, the cladding diameter is 125 µm. A primary coating protects the glass during manufacture, routing and handling. In many cables, fibre with this coating measures approximately 250 µm.
A 900 µm fibre has an additional buffer layer. It may be a tight or looser construction depending on the cable and application. The appropriate layers are stripped from the working area before fusion splicing, but the protector must accommodate the transition between the prepared glass and the coating left on both sides.
250 µm and 900 µm compared
| Feature | 250 µm fibre | 900 µm fibre |
|---|---|---|
| Construction | 125 µm glass with primary coating | fibre with an additional buffer to about 900 µm |
| Typical use | multi-fibre cables, tubes and high-density trays | pigtails, indoor cables and easier identification or handling |
| Space at protector entry | less; micro and mini series can be considered | more; protector inner dimension must be checked |
| Typical selection risk | insufficient length or rigidity | entry opening too small for the retained buffer |
These are nominal values. Coatings, tolerances, buffer stripping and individual cable geometry vary. Where dimensional clearance is limited, measure a sample or perform a trial installation.
Selecting a protector for 250 µm fibre
The most compact V-Protect series are available for 250 µm coated fibre. V12 and V15 are designed for applications where a small recovered diameter is important. They can be useful starting points for micro trays, optical modules and high-density installations.
The smallest protector is not automatically the best. Check the prepared length, tray holder and required rigidity. Where more internal clearance is required, compare V16 and the following series.
Selecting a protector for 900 µm fibre
For 900 µm buffered fibre, pay particular attention to catalogue dimension d, the entry opening before recovery. Series with a 1.2 mm opening can be starting points, including V18, V20, V22 and larger outer-diameter groups.
Do not assume that every construction called 900 µm will fit any protector described for that value. Check the actual diameter of the retained sections, the stripped-buffer length and how the splice will be centred.
Parameters that matter beyond fibre diameter
- d — entry opening before recovery; it must accept the prepared construction;
- D — recovered outer diameter; it must fit the holder and tray clearance;
- L — recovered length; it must cover the splice area and fit the slot;
- Dp and reinforcing-element design — factors affecting rigidity;
- installation time and temperature — product and equipment must be compatible;
- standards, tests and documents required by the project.
Selection process step by step
- Identify the fibre and cable construction.
- Measure the actual diameter of the sections retained near the splice.
- Check the prepared length on both sides.
- Measure the holder, slot and clearance in the intended tray.
- Select a series group using d, D and L.
- Compare rigidity and installation method.
- Verify the exact variant in the current data sheet and perform a trial.
Frequently asked questions
Is 250 µm the glass-fibre diameter?
No. In a common telecommunications fibre, the glass cladding is 125 µm. The 250 µm value includes the primary coating around the glass.
Can a protector for 900 µm fibre be used with 250 µm fibre?
Many larger-opening series cover both constructions, but compatibility must be confirmed for the exact variant, tray and required rigidity. An oversized protector may be unnecessary in a high-density cassette.
Why measure the fibre when the cable supplier specifies 900 µm?
The value is nominal, and buffer construction and tolerances vary. The length of buffer retained inside the protector also affects the installation.
Where can all V-Protect series be compared?
The product catalogue lists dimensions and available variants. The broader selection process is explained in How to choose a fibre optic splice protector.
Need to confirm a selection?
Send the fibre type, actual diameter, holder dimensions and required protector length. The V-Protect team can help identify a suitable series or variant for further assessment.
