What handset pull force should explosion-proof telephones withstand?

A rugged Ex phone can still fail because of a weak handset cord. One hard tug can cut audio, break the hookswitch, and stop emergency calling.

Most explosion-proof telephones should withstand at least 200 N handset pull force for heavy industry. High-abuse areas often target 300 N, while 150 N fits light-duty indoor zones with controlled access.

How to specify handset pull force for Ex telephones

Pull force is a mechanical safety spec, not a “nice to have”

Many buyers focus on gas group, T-rating, and IP66. That is correct, but the handset system is still the most touched part of the product. The cord is pulled by operators wearing gloves. It is also pulled by forklifts, hoses, and tools. In heavy industry, the cord sees more abuse than the enclosure.

A pull-force spec protects three things at once:

• Uptime: fewer cord breaks, fewer intermittent audio issues, fewer service calls.

• Safety: the phone stays usable during alarms and emergency response.

• Maintenance cost: cord replacement is a common “consumable” event, so stronger design reduces downtime.

Separate static pull from real-world jerk loads

A lab pull test is usually a steady tensile load. Real sites apply a fast jerk. A short jerk can create a higher peak load than a slow pull. That is why a 150 N steady rating can still fail in a loading rack. Many projects pick 200 N or 300 N because it gives margin for shock-like pulls.

Make the spec “complete” by including the weak links

A handset pull force number is meaningless if the test ignores:

• the strain relief

• the gland or cord exit

• the internal anchor

• the handset connector

• the hookswitch area

A good requirement defines the test points and the acceptance criteria.

A simple pull-force selection table

Site condition	Typical abuse level	Practical pull-force target	Why this target works
Indoor Zone 2 corridors, controlled access	Low	≥150 N	Most pulls are accidental, not violent
Refinery units, tank farm gates, process decks	Medium to high	≥200 N	Gloves, fast handling, and tool impact are common
Loading racks, shipyards, public-facing points	High	≥300 N	High chance of intentional or accidental hard tugs
Vandal or extreme abuse locations	Very high	≥300 N + design upgrades	Force alone is not enough without anti-twist and strong anchors

The “best” number depends on how hard it is to service the phone

In many Zone 1 areas, opening an Ex d enclosure ¹ is slow and controlled. That makes cord failure more expensive than the cord part itself. In those cases, it is smarter to over-spec the pull force and simplify replacement methods.

The next sections break down pull-force values, test methods, design details, and a maintenance plan that keeps handset problems from becoming a site habit.

If the goal is fewer failures in year two and year three, the pull-force story must be written like a system requirement, not a single line on a datasheet.

A stronger cord spec also helps suppliers build a stable BOM. That means fewer surprises between batches and fewer “same model, different cord” problems during expansion projects.

What tensile load should the armored handset cord meet—≥150N, 200N, or 300N for heavy industry?

A handset cord that looks armored can still fail inside. The outer steel may survive, but the core conductors can break and cause random audio drops.

For heavy industry, 200 N is a solid minimum tensile load target for the handset cord assembly. 300 N is a safer target for loading racks, shipyards, and high-abuse points. 150 N fits low-abuse indoor areas, but it is often too low for outdoor industrial use.

Use a risk-based rule instead of guessing

A pull-force number should match the abuse risk and the service cost. In field projects, the phone that fails most is the one placed at the edge of a busy area. People grab it while walking. They pull while wearing thick gloves. They also drop the handset and let it swing.

A simple rule works:

• Choose 150 N only when access is controlled and the cord is rarely stressed.

• Choose 200 N for most refinery, tank farm, and terminal outdoor points.

• Choose 300 N when the handset can be yanked hard or used by the public or contractors.

Decide whether you are specifying cord-only or full assembly pull force

Some vendors quote a cord tensile value that only applies to the cord sample, not the installed phone. The site needs the full assembly value. The test should be performed on the handset and cord as installed, including strain relief and internal anchor.

Consider “pull angle” and “drop height”

The cord is not always pulled straight. Real pulls happen at angles. That creates bending at the cord exit and at the handset entry. If the spec only tests straight pull, it misses the real failure mode.

It helps to request:

• straight pull test at the cord exit

• angled pull test that loads the strain relief ²

• a drop test for the handset to simulate swing and impact

A practical procurement table for tensile load

Requirement style	What it should say	What it avoids
Minimum baseline	“Handset cord assembly tensile load ≥200 N”	Under-spec for heavy industry
High abuse	“Handset cord assembly tensile load ≥300 N”	Random failures at loading racks
Test clarity	“Test at handset end and phone end, with strain relief installed”	Cord-only claims that ignore weak joints
Pass criteria	“No loss of audio, no conductor open, no jacket tear”	Cosmetic-only pass that still fails later

My field view on cost and value

In B2B projects, the cost difference between a 200 N and a 300 N handset system is often small compared to one service visit. The bigger cost is permit time, access time, and the system downtime. For Zone 1 phones, it is common to specify 300 N in high-traffic points even when 200 N looks “enough” on paper.

The next step is to make sure the test method matches the claim. A strong number with a weak test is still a weak spec.

Which test methods apply—IEC 60068-2-21, EN 50102/IK10, and cyclic pull tests on hookswitch?

Many buyers see IK10 and assume it covers everything mechanical. It does not. IK is impact. Pull force is a different stress.

Handset pull force and cord anchoring should be verified with tensile and termination tests such as IEC 60068-2-21. IK ratings like IK10 address impact resistance, not tensile pull. For real reliability, add cyclic pull and hookswitch cycling to simulate daily use and abuse.

Use IEC-style tensile tests to validate terminations and anchors

A good tensile test checks the termination strength and the strain relief design. IEC 60068-2-21 ³ is often used as a method reference for mechanical strength of terminations and leads. It is useful because it forces the vendor to define:

• the applied load

• the duration

• the direction

• the acceptance criteria

If the vendor only supplies a generic statement, ask for the test setup and photos. A strong vendor can show exactly where the force was applied and what failed modes were checked.

Understand what IK10 can and cannot prove

IK10 helps confirm the handset and housing survive impact. This matters for harsh sites. Still, IK10 does not prove that the cord withstands a pull. It also does not prove that the internal anchor can handle repeated yanks.

A practical spec uses both:

• IK rating ⁴ for impact abuse

• tensile pull rating for cord and anchor strength

Add cyclic pull because “one-time pull” is not real life

A one-time pull test can pass while the cord fails after months of repeated stress. Cyclic pull tests are valuable because they simulate:

• repeated yanks by operators

• vibration and movement

• micro-damage to conductor strands

A cyclic pull test should specify:

• load level (often lower than max)

• number of cycles

• cycle rate

• and functional monitoring during and after the test

Hookswitch cycling matters more than most teams expect

The hookswitch and cradle area see constant use. A heavy handset drop can shock the hookswitch. A strong cord can transfer more force into the hookswitch area if the design has no damping. That is why hookswitch testing should include:

• standard on/off-hook cycle counts

• off-axis handset placement

• and recovery checks for SIP registration and audio

A test plan table that fits most industrial tenders

Test item	What it proves	Suggested acceptance focus
Tensile pull (static)	Strength of cord anchor and terminations	No electrical open, no anchor slip
Angled pull	Strain relief effectiveness	No jacket tear, no internal damage
Cyclic pull	Long-term fatigue resistance	No intermittent audio, no conductor break
IK impact	Abuse and accidental drops	No housing cracks, handset stays functional
Hookswitch cycles	Daily reliability	Stable detection, no stuck state

A phone that passes this test set is far less likely to become a maintenance headache. The next section explains how design details like Kevlar cores and swivel glands make those results repeatable.

How do strain relief and cord materials affect durability—stainless armor, Kevlar core, swivel glands, and anti-twist?

A thick cord is not automatically a durable cord. Most failures happen at the ends, where bending and twisting concentrate stress.

Durability depends on strain relief and core design, not only armor. Stainless armor protects against cuts and abrasion. A Kevlar core carries tensile loads. Swivel glands reduce twisting. Anti-twist design keeps conductors from winding up and breaking near the handset and enclosure exits.

The real failure point is the first 30 mm at each end

In harsh environments, the cord flexes most at:

• the handset entry

• the enclosure exit

If strain relief is weak, the cord bends sharply. That creates conductor fatigue. Armor can hide this problem until the core breaks.

A strong design adds:

• a long, tapered strain relief boot

• controlled bend radius

• an internal anchor that transfers tensile load to the chassis, not to solder joints

Stainless armor: good for abuse, not enough for tensile strength

Stainless armor helps when the cord:

• rubs against metal edges

• sees chemical washdown

• is dragged or stepped on

Still, armor alone does not guarantee tensile load capacity. The tensile load should be carried by a dedicated strength member or an anchor design.

Kevlar core: the quiet feature that improves pull-force performance

A Kevlar ⁵ strength member can carry tensile load and protect the copper conductors from being the “rope.” This improves both pull strength and fatigue life. It also helps during cyclic pull because the conductor strands are not stretched as much per cycle.

A common design goal is:

• Kevlar core takes the pull load

• copper conductors carry signals and power

• armor protects the jacket from abrasion

Swivel and anti-twist reduce real-world torsion failures

Twist is a common hidden killer. Operators rotate the handset. The cord winds up. Then the conductors break near the ends.

Two features help a lot:

• Swivel gland or swivel cord entry: allows rotation without twisting the internal conductors.

• Anti-twist handset connector design: reduces torsion transfer into the cable core.

Do not forget chemical and UV resistance at the cord jacket

On outdoor sites, the cord jacket faces UV, salt, and cleaning fluids. A cord can pass a pull test and still crack after a year if the jacket is not stable. That is why a durability spec should include:

• material resistance to oils and cleaners

• UV and ozone resistance if exposed

• and temperature range stability

A design feature table you can use during vendor selection

Design feature	What it improves	What to check in drawings or samples
Internal anchor point	Prevents pull load on PCB or solder joints	Anchor to chassis with defined clamp
Long strain relief boot	Reduces bend stress at ends	Boot length and stiffness feel
Kevlar strength member	Raises tensile and fatigue life	Vendor statement + cutaway sample
Stainless armor	Protects from abrasion and cuts	Armor braid quality and end termination
Swivel gland	Reduces torsion and twist damage	Smooth rotation and sealed design
Anti-twist routing	Prevents conductor wind-up	Cord does not coil under normal use

A strong cord system is a design package. It is not one material choice. The final section ties this to warranty and maintenance, because cords and handsets are still field-wear items even with the best design.

What warranty and maintenance practices cover handset/cord replacements—MTBF, spare parts kits, and field-swappable modules?

If the handset or cord fails, the site does not care about the phone’s MTBF. The site wants the fastest safe fix with minimal permits and downtime.

The best warranty plan treats the handset and cord as serviceable wear parts. It uses clear replacement coverage terms, spare kits sized to site risk, and field-swappable modules that reduce Ex enclosure opening. MTBF should be used for the electronics, while handset/cord life is managed with spares, inspection, and simple replacement procedures.

Separate electronics MTBF from mechanical wear life

MTBF ⁶ is useful for core electronics like CPU, power, and network parts. Handsets and cords live in the mechanical world. They wear with use, abuse, chemicals, and UV. A clean maintenance plan keeps these topics separate:

• MTBF for electronic subsystem reliability planning

• service life and replacement planning for cord and handset assemblies

Warranty language should be clear on what is covered

A strong vendor warranty statement should explain:

• whether cords and handsets are covered for defects

• what counts as misuse or abuse

• how replacement is handled in hazardous areas

• and whether spare parts are available for the full product lifecycle

In many heavy industry projects, a practical expectation is that cords are supported as spare parts for years, because the phone may stay installed for a long time.

Spare parts kits reduce downtime more than any brochure feature

A good spare kit for a site can include:

• handset + cord assembly

• strain relief boots

• gland seals and washers

• hookswitch spare module (if modular)

• and a small hardware pack for mounting and bonding

The quantity depends on risk. A simple rule works:

• low-abuse areas: 1–2% spares

• heavy-abuse areas: 3–5% spares

• critical emergency networks: spares stored on site with clear ownership

Field-swappable modules are a big win in hazardous areas

Opening an Ex d enclosure can require permits ⁷ and gas-free conditions depending on site rules. A design that allows external replacement of the handset cord, or a modular internal assembly that is quick to service, reduces downtime.

Useful vendor features include:

• accessible cord termination without deep disassembly

• quick swap handset assembly with keyed connectors

• clear torque and sealing instructions to preserve IP and Ex integrity

Maintenance practices that prevent small cord issues from becoming failures

A simple inspection routine helps:

• check cord jacket for cuts and cracking

• check strain relief for hardening and tears

• check gland tightness and seal condition

• check handset seating and hookswitch response

• run a short call test, including audio and DTMF ⁸ if used

A warranty and maintenance table for B2B buyers

Item	What to request	Why it matters
Warranty scope	Clear terms for handset and cord	Prevents disputes during year one failures
Spare parts availability	Multi-year spare availability promise	Supports long project lifecycles
Spare kit contents	Handset, cord, seals, small hardware	Reduces downtime and site improvisation
Field swap method	Procedure and time estimate steps	Reduces permit and labor impact
Service documentation	Torque, sealing, and inspection checklist	Protects IP rating and Ex integrity

A handset pull-force spec is not complete until warranty and spares are planned. Heavy industry does not forgive downtime. A phone that is easy to service becomes the phone the site trusts.

Conclusion

Target 200 N for most heavy industry and 300 N for high-abuse areas, then validate with tensile and cyclic tests ⁹, strong strain relief, and a spare-kit plan that keeps replacements fast and safe.

---

Footnotes