"Cable Cancer": The Green Goo Hiding Behind Old Sockets
I rushed back from a job to get this post written as it is very important to highlight. I was on a job today for what sounded like a straightforward switch fault. Heard arcing, assumed it was a dodgy switch - the kind of thing I see most weeks. Pulled it apart and found something else entirely: a bright green, slightly waxy residue coating the copper underneath. I call it cable cancer, because that's honestly what it looks like once you've seen it a few times, something spreading quietly inside the wiring, years before it ever announces itself.
The proper name for it is far less dramatic: electricians usually call it green goo or green slime, and it's a well-documented issue, not something unusual to your house specifically.
What's Actually Happening
Cables made roughly from the mid-1960s to the early 1980s used a plasticiser, a chemical additive that kept the PVC insulation flexible called di-isoctyl phthalate. Over years of use, especially anywhere the cable heats and cools repeatedly, that plasticiser slowly migrates out of the insulation and reacts with the copper conductor underneath it. The reaction produces a green residue, chemically similar to verdigris, the same green coating you sometimes see on old copper pipework or coins.
It shows up most often at switches and socket outlets, because on a vertical cable run, gravity pulls the plasticiser downward over the years until it collects at the lowest point - which is usually exactly where a switch or socket sits.
Why It Matters, Not Just What It Looks Like
The residue itself isn't the danger. What it tells you is the actual concern: the insulation has been slowly breaking down for years, quietly, with nothing visible from the front of the switch to warn you. Industry guidance treats it as a genuine sign that a circuit's insulation is degrading, and the standard recommendation once it's found is to plan a rewire of the affected circuits — not just clean it off and put the faceplate back on.
There's also a very good reason not to touch it with bare hands if you ever spot it yourself. It can be mildly irritant on skin, and in some conditions it can become slightly conductive, which is part of why what I heard today as "arcing" wasn't actually a switch fault at all.
What Today's Job Actually Looked Like
The initial call was a switch that had started crackling. On the surface, that's routine - a loose connection, a worn switch, ten-minute fix. Behind the faceplate, the insulation on both conductors had degraded enough that the green residue had built up and was very likely the actual source of the arcing sound, not the switch mechanism itself. The switch was the symptom. The cable was the cause.
Cable degradation isn't the only thing that gets missed until someone opens the board. Here's a real example of the kind of DIY faults we regularly find behind an old consumer unit.
Why This Might Explain a Local Pattern
Both Salfords and Earlswood have a lot of housing built in the 1970s and 80s - exactly the era this affects. It's a genuine possibility that some of the EICR enquiries we see from those areas are connected to this, even when the original call-out reason is something else entirely, like a crackling switch or a socket that's started smelling faintly warm. If your property falls in that build era and you've never had the wiring properly inspected, it's worth knowing this is one of the things a proper EICR is specifically checking for.
What to Do If You See It
If you ever take a faceplate off or an electrician does and see a green residue on the wiring behind it:
Don't touch it with bare hands. Gloves, or leave it for a professional.
Don't assume it's just cosmetic. It's a sign of insulation degradation, not surface dirt.
Get the affected circuit properly assessed. A single instance might mean a small remedial job. Multiple instances, or cable from this era throughout the property, usually points toward a fuller rewire conversation.
If your property was wired any time from the 1960s through the early 1980s and you're not sure what condition it's actually in, an EICR is the way to find out before it turns into an unplanned emergency call-out. If you already know it needs more than a spot repair, our rewiring page covers what a full or partial rewire actually involves.
Frequently Asked Questions
What is the green goo found in old electrical wiring? It's commonly known as "green goo" or "green slime" in the electrical trade. It's caused by a plasticiser chemical in older PVC cable insulation migrating out over time and reacting with the copper conductor, producing a green residue similar to verdigris.
Is green residue on electrical cables dangerous? The residue itself is generally low-toxicity, but it's a sign that the cable insulation is degrading. Left unaddressed, it can lead to reduced insulation resistance and, in some cases, contribute to arcing or short circuits. It should be assessed by a qualified electrician rather than ignored.
Which properties are affected by this issue? It's most commonly found in cables manufactured from the mid-1960s to the early 1980s. Properties wired or last rewired within that period are the most likely to be affected, particularly at switches and socket outlets on vertical cable runs.
Can I just clean off the green residue myself? No. It shouldn't be handled with bare hands, and cleaning it off doesn't address the underlying cause - the insulation has already started to degrade. The affected circuit needs proper assessment, not just a wipe-down.
Does finding this mean I need a full rewire? Not always. A single, isolated instance may only need remedial work on that circuit. If it's found in multiple locations, or the property's wiring throughout is from the affected era, a fuller rewire is the more realistic long-term fix. An EICR is the way to establish which situation you're actually in.
