We’d just finished setting the plug on a pump down perforating job—stage one had gone off perfectly. Guns fired, no hang-ups, everything checked out on the wireline control system. But on the next stage, just as we went to arm the string, the system showed an open circuit.

Now, any wireline engineer will tell you an “open” post-firing isn’t a good sign. It usually means one thing: something broke. And sure enough, when we pulled the string and broke it down at the shop, we found the culprit—a broken through wire, snapped right at the post of the EB switch on the bottom perforating gun.

That one break shut down a perfectly good run and reminded us how easy it is to damage gun wiring during build-up. Since then, we’ve changed how we assemble switch subs and through-wired guns—and we haven’t had the same failure since.

The Real Risk Hiding Inside the Switch Sub

This wasn’t a rare freak failure. If you’ve ever built perforating systems for cased hole wireline, you know that inside every gun barrel lies a fragile copper wire or conductive post that connects each switch to the next. That tiny through wire carries the electrical pulse needed to fire each perforating gun in sequence.

The failure came from how the gun was built—not from any faulty component. During assembly, the crew screwed the switch sub into the barrel without controlling wire tension. As the parts threaded together, the wire twisted tight until it snapped right at the contact post.

The worst part? The switch passed the initial test. The break didn’t show itself until after plug setting—when it was too late to fix without pulling the string.

Standardizing the Fix: Gun Switching Best Practice

After this misrun, we implemented a new gun building SOP focused on protecting that fragile wire during assembly. These steps are now standard on every wireline perforating job we do, and they’ve become part of how we teach new techs in our wireline courses.

Through-Wire Switching Procedure (Standard SOP)

1. Apply Tension to the Through Wire During Assembly

• When screwing the switch sub into the gun barrel, a second person must hold the wire to keep it under mild tension.

• This keeps the wire straight and prevents it from wrapping, pinching, or twisting as the barrel rotates.

2. Back-Spin the Sub Before Final Thread Engagement

• Before screwing the gun barrel onto the sub, back-spin the sub 8 full turns.

• This compensates for twist and prevents the wire from torquing inside the barrel.

3. Allow Slack Before Final Tightening

• When the sub is two turns from being tight, release the wire to allow a small amount of slack.

• This slack absorbs mechanical shock from plug setting, gun firing, and downhole jarring.

These changes ensure the electrical continuity between guns remains intact and protect against premature open circuits on wireline gun strings.

Why It Matters in Cased Hole Logging

In the wireline services oil and gas world, especially with cased hole well services, there’s no room for error in the firing line. A single broken wire can:

• Halt an entire perforation stage

• Lead to unintentional fishing wire line events

• Result in delayed formation evaluation

• Cause misalignment in perforating systems and miss-targets in the zone

In an industry where wireline companies are expected to deliver complete wireline solutions on tight frac schedules, this kind of small error has a big ripple effect.

Reinforcing the Lesson in Wireline Courses

This best practice is now a core part of our training programs. In every wireline course, we teach crews how to:

• Identify at-risk components in gunstring assemblies

• Properly support and route through wires

• Use back-spinning techniques to protect wire integrity

• Spot signs of strain, torque, and twist before the gun goes in hole

Crews also learn how to read dumpfiles and interpret gun diagnostics so they can tell the difference between a real open and a software fault.

How This Applies Across the Wireline Business

This isn’t just for pump down perforating. We now use this SOP across all high-reliability wireline services, including:

• Multi-stage well perforation

• Precision wireline perforating gun sequencing

• Pressure-sensitive cement bond log (CBL log) runs

• Long-string production logging services

• Fishing jobs and pipe recovery operations where gun misfires increase retrieval risks

Final Thoughts – Small Wire, Big Impact

It’s easy to focus on big tools—logging cable, pressure control equipment, perforating guns, and wireline trucks. But sometimes it’s the thinnest copper wire in the string that determines the outcome of a job.

This failure taught us that attention to detail during tool assembly matters just as much as execution downhole. And it’s why we’ll never again build a gunstring without checking, tensioning, and protecting the through wire.