MPO cable bend radius and pulling tension

The limits in one table

Most damage to an MPO trunk happens in the hour it is installed, and most of it comes from two numbers that were not checked: how tightly the cable was bent and how hard it was pulled. The table puts the usual rules side by side. Where a figure comes from a standard, the source is named. Where it comes from a data sheet, you must take the value from the data sheet of the cable you buy.

The rest of the page explains each row, with a worked pull at the end. For the general bend radius rules that apply to all fibre cable, see the bend radius for fibre cable guide. This page is about the specific case of pre-terminated MPO and MTP trunks, which are bulkier, carry many fibres and end in connectors that cannot be pulled.

Bend radius: what the number protects

A bend radius limit protects two things: the glass and the optical performance. A bend that is too tight can break the fibre, and it takes only a modest bend to add loss. The bend radius is therefore stated in two conditions.

The static radius (10 times the outside diameter, no load) applies after the cable is installed and resting in its final path. The dynamic radius (20 times the outside diameter) applies while the cable is under tension during the pull, because the strength members carry the load and the fibres are squeezed against the outside of the bend. A cable that is acceptable in a 45 mm bend once it is installed may need a 90 mm bend while it is being pulled around that corner.

The outside diameter that counts is the diameter of the cable you hold, not the diameter on the order. A trunk with a pulling sleeve, a thicker section at the break-out, or a bundle of several trunks tied together has a larger effective diameter and needs a larger radius. When trunks are bundled in a tray, the radius that matters is the one for the bundle at the bend.

Bend radius also feeds the loss budget. High-speed multimode links such as 100GBASE-SR4 have a small channel loss allowance of about 1.9 dB on OM4, as covered in the parallel optics cabling guide. Every tenth of a dB lost to a tight bend leaves less for connectors. The mechanism of bend loss is explained in the macrobend and microbend loss guide.

Why the construction of the trunk changes the answer

MPO trunks are built in several ways. A cable with 12 or 24 fibres may hold the fibres as loose coated fibres in a tube, as bonded groups, or as a stack of ribbons. Their bending behaviour differs.

• Ribbon cable. A ribbon is a flat row of fibres. It bends easily across its thickness and resists bending across its width. A bend that twists the ribbon edge-first loads the outer fibres unevenly. Cables with stacked ribbons are built to handle normal routing, but a hard kink at a tray edge can damage them.
• Loose-tube or sub-unit cable. Fibres lie loose in a tube or in tight sub-units. The cable bends in any direction with similar behaviour.
• Small-diameter, high-density trunk. The thin jacket saves space but gives less protection, so the permitted tension is lower. See the high-density MPO trunk cable page.
• Armoured trunk. Steel armour makes the cable stiff and heavy. The bend radius is larger, but the pulling tension is also higher. The armoured MPO trunk cable page describes the trade-offs.

Always take the bend radius and tension rating from the data sheet of the exact cable. Ask for it with the quote. The values for ribbon, loose-tube and small-diameter cables with the same fibre count can differ by a factor of two.

Breakout legs, boots and the connector

The weak part of an MPO assembly is the connector end. The legs of a breakout (the 2.0 mm or thinner cables to LC connectors) follow the bend rules of a small cable, and the radius that is safe for them is much smaller in millimetres than for the trunk. The fan-out joint, where the trunk divides into legs, is a stiff point and should not be bent.

The MPO or MTP connector has a plastic housing and a strain relief boot. The boot is designed to guide the cable out of the connector in a straight line. The pull on the cable has to be taken by the cable itself, and never by the connector. Side loads on a boot, which happen when a heavy trunk hangs from a rack port at an angle, can push the ferrule off alignment and raise loss. Leave a straight section of cable behind each connector, and support the cable close to the rack with a clamp or tie bar. The fan-out lengths and leg choices are discussed in the MPO fan-out lengths and leg types guide.

Cleaning and inspection are separate tasks and must happen before every mating. The guide to cleaning MPO connectors shows the method.

Pulling tension: where the force goes

A cable is pulled by its strength members, which are normally aramid yarn or glass rods under the jacket. The jacket and the fibres should carry nothing. A pulling eye or pulling grip transfers the force from the installer's rope to the strength members. A pre-terminated trunk with a pulling eye is built so the connectors sit inside a protective sleeve, with the eye attached to the strength members, as on the MPO trunk cables with pulling eye page.

Three tension values appear on data sheets, and they are different:

1. Maximum pulling tension (short term). The load the cable can bear during the installation, with the bend radius at its dynamic value.
2. Maximum long-term tension. The load the cable can bear when it is installed and hanging, such as in a vertical riser. This is lower than the pulling tension.
3. Pulling eye rating. The load the eye and its attachment can take. If this is below the cable value, it is the effective limit.

Friction raises tension as the cable goes around bends in a duct or conduit. The tension increases by a factor of e raised to the power of the friction coefficient times the bend angle in radians, so it grows exponentially with the number of bends. This is the reason why routes with many corners are dangerous.

Worked example: a pull through conduit

All values are assumed to illustrate the method. Use the cable data sheet and a measured route.

A small-diameter 12-fibre OM4 trunk has an outside diameter of 4.5 mm and a mass of 25 g per metre. Its data sheet gives a bend radius of 10 times the diameter unloaded (45 mm) and 20 times loaded (90 mm), and a short-term tension limit of 100 N. The conduit run is 75 m long with four 90-degree bends, so five straight sections of 15 m. Assume a friction coefficient of 0.4 with no lubricant.

• Weight per metre as a force: 0.025 kg x 9.81 = 0.245 N/m.
• Friction force on each straight 15 m section: 0.4 x 0.245 x 15 = 1.47 N.
• Multiplier at each 90-degree bend: e to the power of 0.4 x 1.5708, which is about 1.87.

Working from the start of the pull: after section 1, 1.47 N. After bend 1, 2.76 N. After section 2, 4.23 N, after bend 2, 7.93 N. After section 3, 9.40 N, after bend 3, 17.6 N. After section 4, 19.1 N, after bend 4, 35.8 N. After section 5, the final tension at the pulling eye is about 37 N. That is well inside the 100 N limit.

Now add three more 90-degree bends, each followed by a 15 m section, which is a route with seven corners. The tension climbs to about 255 N. That is two and a half times the data sheet limit, and the cable or the eye would fail or be damaged. Three extra corners changed a safe pull into a failed one. The cure is not a stronger rope. It is a lubricant, a shorter pull, pulling from the middle toward both ends, or an intermediate pull box so that no single pull has more than three or four corners.

On the route: trays, risers and bundles

• Waterfall edges. The edge of a cable tray where the cable drops over has a fixed radius. Use a waterfall fitting or a bend-limiting guide that matches the dynamic radius of the cable.
• Vertical runs. Support the trunk at intervals so its own weight does not sit on one clamp or on the connector. The long-term tension limit applies here.
• Cable ties. Tighten ties only until the cable is held. A tie pulled hard enough to dent the jacket crushes the fibres inside. Hook-and-loop straps are kinder and easy to adjust.
• Bundles. A bundle bends as one large cable. Use the radius of the bundle, not the radius of one trunk.
• Mixed cable. Heavy copper or armoured cable stacked on top of a thin trunk crushes it. Run light cable on top.
• Temperature. Cold jackets are stiffer. See the fibre cable temperature ratings guide for the installation temperature range.

Pre-pull checklist

1. Read the bend radius and tension limit from the data sheet and write them on the work order.
2. Check the route for the number and the radius of bends, and compare each with the dynamic radius.
3. Agree the pulling method. Hand pulling with a tension gauge is better than a powered winch for small trunks.
4. Fit the pulling eye on the strength members. Do not tie the rope to the connectors.
5. Clean the route of sharp edges and snags before the pull.
6. Put an observer at each bend.
7. Pull at a steady, slow speed and stop if the cable resists.
8. After the pull, inspect and clean the connectors, then test the links. The MPO trunk cable installation checklist gives the full sequence.

Common mistakes and when to specify differently

Using the static radius for the pull. The 10 times figure applies to a resting cable. During the pull it is 20 times.

Pulling a bundle by one cable. Only one trunk carries the load, and it may exceed its limit while the others hang along for the ride.

Pulling by the connector. Always use the eye on a trunk ordered for pulling. If the trunk was ordered without one, the connectors are not designed for pulling at all.

Assuming armour fixes everything. Armour raises crush resistance, and the cable stays sensitive to bends and to tension at the connector transition.

If the route has many bends, long pulls or vertical sections, specify a pulling eye, a stronger construction, or a shorter trunk joined at a mid-route cassette. The guide to choosing MPO trunk cable length helps with the length, and the trunk cables page lists the range. When you request a quote, describe the route, the number of bends and the pull length, and ask for the bend radius and tension data of the offered construction. Send it through the request a quote page. Radix makes MPO and MTP assemblies to the buyer's drawing in Pune.