GRP armoured fibre optic cable uses rigid glass-reinforced plastic rods, or a GRP layer, instead of steel tape or wire to protect the core. It carries no current, needs no earthing and suits routes near high-voltage plant or lightning exposure. It resists pulling and gnawing better than it resists crushing, so steel armour is still the choice for heavy crush loads.
What GRP means in a fibre cable
GRP stands for glass-reinforced plastic. Glass fibres are bonded in a resin and pulled into a solid rod, which is stiff along its length and has no metal in it. The same material is sold as FRP (fibre-reinforced plastic), and cable data sheets use both names for the same idea. In a fibre optic cable the rods do two jobs at once: they carry the pulling load so the glass fibres inside do not stretch, and they give the sheath a hard layer that is difficult to bite through.
Searches for "grp armoured fibre optic cable" mix up two different constructions, and it helps to separate them before you ask a supplier for anything.
- GRP strength members. One or more rods sit beside or around the core, under a plastic sheath. This is common in flat drop cables and in many outdoor cables. The rods give tensile strength and keep the cable from shrinking in cold weather. They are not armour in the sense of a protective layer.
- GRP armour. A layer of rods or a rigid GRP sleeve runs all around the core, under the outer jacket. The layer is intended to resist rodent bites and impact, as a metal-free substitute for steel wire or steel tape.
Both are called "GRP armoured" in the market. When you write a specification, say which one you mean, and say what the cable must survive: pull, bite, crush, or only the absence of metal.
This page sits inside our wider armoured fibre optic cables range. For the general case against unarmoured cable, start with the armoured vs unarmoured fibre cable guide, then come back here for the non-metallic option.
Why a route calls for no metal at all
Steel tape and steel wire are the default armour, and they work well. A metal layer has side effects, though, and in some places those side effects are the problem. The reasons to pick GRP come down to four situations.
| Situation | Why metal is a problem | What GRP changes |
|---|---|---|
| Route runs alongside high-voltage lines or inside a substation compound | A metal armour layer can pick up induced voltage along its length | Rods are dielectric, so nothing is induced and nobody has to bond or earth the cable |
| Open ground with frequent lightning, such as a hill-top mast route | A long metal sheath can carry a surge into the equipment room | No conductive path through the sheath |
| Hazardous area where the site rules limit conductive items | Bonding every armoured cable adds design work and inspection points | One less item in the bonding scheme |
| Pulls through long ducts, or routes where cable weight matters | Steel tape adds weight and stiffness | Rods carry the pull at a lower weight per metre |
The electrical point is the one with a rule behind it. In the US National Electrical Code, optical fibre cable that has no metallic members, such as armour or strength members made of metal, is classed as non-conductive, and cable with metal is classed as conductive. Conductive cable has to be bonded and grounded where it enters a building. Non-conductive cable does not. Other countries use their own wiring rules, so check the standard that applies to your site, but the same principle holds nearly everywhere: removing the metal removes the bonding job.
Steel is still the better answer in several places, and a later section sets out when.
What the rods do and do not protect against
It is easy to assume that "armoured" means one thing. It does not. Each threat acts on the cable in a different way, and GRP deals with some threats much better than others.
Tensile load
This is where GRP is strongest. A rod of glass-reinforced plastic has a high tensile modulus and stretches very little, so a cable with rods can be pulled through a duct or hung between supports with the fibre kept near zero strain. Cable standards test this with a defined tensile load (the IEC 60794-1 series of optical cable test methods sets out the method), and the data sheet gives a short-term and a long-term rating in newtons. Compare those two numbers. Short-term is the pulling load during installation. Long-term is the load the cable can carry for years once installed.
Rodent attack
Rodents gnaw to keep their teeth worn down, and a sheath with a hard glass layer is unpleasant to chew. Glass-based armour is a recognised answer to this problem and is sold as rodent resistant. Treat that as a design intent, not a promise. Rodent behaviour and tooth strength vary with the species and the season. Steel armour remains the layer people trust most in rodent-heavy ducts, and some specifiers prefer it. If rodents are the main risk, read the guide on rodent-protected fibre cable and ask any supplier what evidence supports the claim.
Crush and impact
Here GRP is weaker than steel tape. Rods are strong along their length, but a heavy point load can break a rod or crack it, and the rods have gaps between them. Corrugated steel tape wraps the whole circumference and spreads a load around the cable. For a route where trucks, rock fill, or a trench backfill can press on the cable, a continuous steel layer gives better crush performance than rods. Cable standards test crush with a specified load on a short length (the IEC 60794-1 series sets out the method), and the data sheet will show the load in newtons per 100 mm. Use that figure, not the word "armoured", to compare cables.
Bending
Rigid rods do not like tight bends. A GRP-armoured cable is stiffer than an unarmoured one of the same outside diameter, and the minimum bend radius on the data sheet is larger as a result. The practical rule of thumb for many outdoor cables is 10 times the outside diameter with no load and 20 times while under pulling tension. Some GRP designs ask for more. Always use the figure printed on the data sheet for the cable you buy, because it varies by construction.
Rods also do not suit repeated twisting, so drum-wound or mobile uses need a flexible construction, as covered in our guide to tactical fibre cable reels and deployment.
GRP against steel tape and steel wire
The following table gives the comparison most buyers need. The statements are general, and individual cable designs vary. Where numbers matter, take them from the data sheet. The longer treatment of armour layers is in our guide to steel tape, steel wire and GRP armour.
| Property | GRP rods | Corrugated steel tape | Steel wire |
|---|---|---|---|
| Conducts electricity | No | Yes, needs bonding | Yes, needs bonding |
| Typical role | Pull load, bite resistance | Crush and rodent resistance | High pull load, often for burial or water crossing |
| Crush resistance | Moderate | High | Moderate to high |
| Weight for the same diameter | Lower | Higher | Highest |
| Lightning and induced voltage | No path | Possible path | Possible path |
| Bend flexibility | Stiff | Moderate | Stiff |
| Corrosion | None | Coated steel can corrode if the jacket is damaged | Same |
Density makes the weight difference real. Glass-reinforced plastic is in the region of 2 g/cm³, and steel is about 7.85 g/cm³, so the same volume of armour in steel is nearly four times heavier. The worked example below applies this with assumed figures.
Worked example: weight and pull on a 600 m duct run
All figures below are assumptions for illustration. Replace them with the data sheet values of the cable you intend to buy.
Route. A 600 m outdoor duct run between two sheds on a utility site, with one straight section and two bends, in a duct that is known to carry rodent activity.
Assumed cables.
| Item | Steel tape armoured | GRP armoured |
|---|---|---|
| Outside diameter (assumed) | 12 mm | 12 mm |
| Mass per kilometre (assumed) | 140 kg/km | 105 kg/km |
| Short-term tensile rating (assumed) | 1,500 N | 2,000 N |
| Minimum bend radius, loaded (assumed 20 x OD) | 240 mm | 240 mm |
Mass on the drum. 600 m is 0.6 km. The steel tape cable weighs 0.6 x 140 = 84 kg and the GRP cable weighs 0.6 x 105 = 63 kg. The difference is 21 kg, which one person can lift but two people will notice when feeding a drum.
Pull force. Pulling tension during installation is roughly the sum of the friction along the duct and the extra friction at the bends. With a friction factor of 0.3 and a straight run, the pull is about 0.3 x 63 kg x 9.81 = 185 N for the GRP cable, with bends adding to that. Even with bends doubling the figure, the pull of about 370 N is a fraction of the 2,000 N short-term rating. The rating is not the limiting factor here. Keeping the pull below the rating still matters, so use a pulling eye with a tension limiter.
Decision. In this example the rodent risk drives the choice. If the cable data sheet shows rodent-resistance evidence the buyer accepts, GRP is lighter, needs no earthing at either shed, and suits the job. If the evidence is thin, steel tape is the safer choice and the bonding at each shed becomes part of the design. Either way the numbers show that the pull is easy, so the choice depends on the threat and not on the load.
Where GRP armoured cable appears in practice
The following routes are where specifiers most often ask for non-metallic armour. In each case the reason is the absence of metal, and the armour layer is a secondary benefit.
- Power utility sites. Fibre between switchyard buildings and control rooms where metallic sheaths would carry ground potential rise during a fault.
- Telecom masts and towers. Down-leads on a tall structure that attracts lightning, where a dielectric sheath keeps surge energy out of the cable.
- Rail trackside. Duct and trough routes beside overhead electrified lines, where induced voltage on a metal sheath is a design concern. The guide on unarmoured vs armoured cable for railway and traffic sets out the other factors.
- Industrial plants with rodents. Cable trays at low level in food or grain buildings, where a metal sheath adds a bonding task and gnawing is a real risk.
Choosing between GRP and steel: decision steps
- List the hazards on the route: pull, rodents, crush, lightning, induced voltage, water, chemicals, heat. Mark each as present, possible, or absent.
- If induced voltage or lightning is present, or if the site bans conductive cable, GRP or another dielectric construction is likely required. Go to step 5.
- If crush from vehicles, rock or heavy backfill is present, ask for a steel tape cable or a design with a documented crush rating. A GRP cable with a low crush figure should be dropped here.
- If rodents are the main hazard and no electrical concern exists, compare steel tape and GRP data sheets, and ask for the test evidence behind any rodent claim.
- Check the tensile ratings against the calculated pull, with a margin. Use the worked example above as a template.
- Check bend radius against the tightest corner in the route. A stiff cable that cannot make the corner is a bad purchase, however strong it is.
- Write the specification with numbers, not adjectives: tensile load in newtons, crush load in newtons per 100 mm, bend radius in millimetres, operating temperature range, jacket material.
Terminating and handling GRP armoured cable
A factory-terminated assembly needs the armour handled at each end, and the details matter more than they seem to. The rods have to be cut and dressed back so they do not poke through the boot, and they must be anchored or the pull on the cable will reach the connector instead of the strength members.
- Strain relief. The tensile load has to transfer from rods and aramid yarn to the connector body or to a pulling eye, not to the fibre. Ask how the rods are anchored at the termination.
- Fanout. At the end of the armoured section, the fibres are protected in loose tube or in individual buffered legs. Specify the leg length from the armour end to each connector. A leg that is too short cannot reach the equipment, and one that is too long loops around the rack.
- Cut ends. Cut GRP rods leave fine glass splinters, so wear gloves and eye protection when trimming.
- Water blocking. If the route is wet, the cable should have water-blocking tape or gel. The armour layer does not do this job.
Armoured assemblies are commonly ordered as patch cords or tails already terminated, which avoids field splicing on a hard sheath. The page on custom fibre optic assemblies explains what to put on the drawing.
When not to use GRP armoured cable
The following are common mistakes, and each one comes from reading "armoured" as a promise of general toughness.
- Heavy crush routes. Under a road, in a trench that will be backfilled with rock, or anywhere a vehicle can load the cable. Use steel tape, or put the cable in a duct designed for the load.
- Tight indoor bends. Rack-to-rack patching, cable tray corners and under-floor routes with small radii. Use a flexible cord instead. The mismatch is also costly, since a GRP cable in a tight corner can kink and break fibres.
- Dynamic flexing. Drum-wound, robotic or festoon applications. Rods do not tolerate repeated bending.
- Direct burial with no duct, where rodents are not the threat. Check the guide on armoured vs direct burial cable. A burial-rated sheath is a separate question from armour.
What to put in the specification
A short specification avoids most arguments. Write each line as a number or a named material where possible.
| Item | Example of what to state | Why it matters |
|---|---|---|
| Fibre type and count | OS2 single-mode, 12 fibres | Sets the loss and the core size |
| Armour | Non-metallic GRP rod layer, no metal members anywhere in the cable | Makes the dielectric requirement explicit |
| Tensile rating | Short-term and long-term loads in newtons | Lets you check the pull calculation |
| Crush rating | Load in newtons per 100 mm | Separates GRP from steel designs fairly |
| Minimum bend radius | Loaded and unloaded, in millimetres | Confirms the cable fits the corners |
| Outer jacket | Material, colour, UV resistance, fire behaviour | Governs where the cable may be run |
| Temperature range | Installation and operating | Plastic stiffens in cold and softens in heat |
| Termination | Connector type, polish, leg length, pulling eye | Defines the assembly drawing |
| Length and marking | Length in metres, tolerance, cable ID labels | Avoids rework on site |
The structure of a full request for quotation is set out in the guide to writing an RFQ for fibre assemblies.
Sending the drawing to Radix
Radix Fibrotech makes armoured and tactical fibre cables and assemblies to the customer's drawing and bill of materials in Pune. Whether a given GRP rod construction can be built depends on the cable and the connector, and we do not assume it here. Send the drawing with the route description, fibre count, connector types, lengths and the data sheet of the cable you want used, through the request a quote page or by phone or WhatsApp, and we reply with what we can build. If you do not yet have a cable chosen, send the hazards list from the decision steps above and ask for options. The wider comparison of armour types will help you review them.
Questions buyers ask
Is GRP armoured fibre optic cable the same as FRP cable?
In most data sheets, yes. GRP and FRP both describe rods or layers made of glass fibre in a resin. Some suppliers use FRP for aramid-reinforced plastic as well, so check the material named in the construction table, and look for the word "glass".
Does GRP armour stop rodents?
It makes gnawing harder because the layer is hard glass, and it is sold as rodent resistant. It is not an absolute barrier. If rodents are the main risk, ask for the supplier's test evidence, and consider a conduit or steel tape cable as a second layer of protection.
Do I need to earth a GRP armoured cable?
No, if the cable has no metal anywhere in its construction. The rods do not conduct, so there is nothing to bond. Check the data sheet carefully, because some cables with a GRP layer still include a metal tape for moisture or a metal strength wire, and those do need bonding.
Is GRP armour as strong as steel tape?
It is strong in tension, often stronger per kilogram than steel, but weaker against crush. Steel tape wraps the whole cable and spreads a load. Compare the tensile and crush ratings on the data sheets instead of comparing the word armoured.
What bend radius should I plan for?
Use the figure on the data sheet. A common rule for outdoor cable is 10 times the outside diameter with no load and 20 times under tension, and GRP cables can need more than that because the rods are stiff.
Can a GRP armoured cable go underground?
It can go in a duct, and many are designed for it. Direct burial depends on the jacket and the crush rating. Look for a burial rating on the data sheet, and see our guide comparing armoured and direct burial cable.
Can it be made as a patch cord with connectors fitted?
Armoured cable can be terminated at the factory with LC, SC, FC or ST connectors, with the armour anchored at each end. Radix works to the customer's drawing, so send the drawing with the cable data sheet and we reply with what we can build.
What lengths and fibre counts are available?
Both depend on the cable construction. Fibre counts for armoured outdoor cable run from a few fibres up to many dozens, and lengths are limited by the drum and the pulling limit. State the route length and fibre count in the quote and ask for the cable options that suit it.