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2
Volume 54. No. 6
June 2025
ISSN 2632 573X
Editorial
Don’t tin wires going into terminal blocks
This seems to be a slightly controversial and misunderstood topic.
There is a temptation to tin the end of a wire before you insert into
into a terminal block, especially a screw terminal. Doing so stops
stray strands from sticking out and makes inserting the wire easier.
However, there are serious problems with doing so, and you really
shouldn’t. In fact, this practice is actively discouraged or even
prohibited by most wiring standards, including BS 7671, AS/NZS
3000, and related IEC and UL standards.
The problems relate to mechanical stability and long-term
reliability. Unlike copper, solder is a soft metal with a relatively
low melting point. Over time, especially in the presence of heat
and pressure, it can ‘cold flow’ – gradually deforming under
the clamping force of a terminal screw. This can lead to a loose
connection, increased contact resistance and, in extreme cases,
overheating or arcing.
Also, when you insert a bare stranded wire into a terminal and
tighten the screw, the strands compress and spread slightly. This
creates a wide, gas-tight contact area, improving both electrical
conductivity and mechanical grip. Tinning prevents that from
happening. The solder locks the strands together, turning them
into a pseudo–solid core that can’t adapt to the clamping surface.
The result is a reduced contact area, less secure grip and higher
resistance.
Solder is also less conductive than copper, so the connection with
the tinned wire not only has a lower contact surface area, but also
higher inherent resistance. This makes for a potentially hot and
unreliable joint.
If you’re dealing with fine-stranded wire or worried about stray
strands, the correct solution is not solder, it’s a ferrule. These are
metal sleeves, usually made of tin-plated copper, that crimp over
the end of a stranded wire to hold it together. A properly crimped
ferrule allows the terminal screw to compress the ferrule itself,
maintaining a secure, stable connection over time. It also preserves
the benefits of a gas-tight contact.
Ferrules are not expensive – decent kits with crimpers and assorted
sizes are readily available. Once you try them, you’ll probably never
go back.
Nicholas Vinen, Electron Publishing (Australia)*
Publisher & Editor,
Practical Electronics Magazine
* a division of Silicon Chip Publications Pty Ltd.
Practical Electronics | June | 2025
