Earthing and surge protection on a cable run between two buildings
A client has two buildings on one site, 60 m apart. Building A has the main comms room and the internet service. Building B has a small office that needs data. There is an existing underground conduit between the buildings, but it also carries a 240 V feed to a sub-board in building B. The client wants you to pull a Cat 6 cable through the same conduit to save trenching. Building B has its own earth electrode, and the two buildings' earth systems are bonded at the main switchboard. The client says "it's all one site, so it's fine."
1. Is pulling the Cat 6 through the shared conduit acceptable? Explain.
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Running communications cable in the same conduit as mains cable is not acceptable. The mains cable can induce interference, and a fault or damage to the mains insulation could put dangerous voltage on the comms cable and anything connected to it. The comms cable must be in a separate conduit with adequate separation.
Even in a separate conduit, a copper cable between two buildings introduces an earth potential difference problem. The two buildings have separate earth electrodes, and although they are bonded at the main switchboard, a lightning strike or a major fault can create a large potential difference between the two ends of the copper cable. That difference drives current through the cable shield and the equipment at each end.
So the answer is no to the shared conduit, and the copper-between-buildings idea needs more thought even in its own conduit.
2. What design would you recommend for the inter-building link?
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Use fibre between the buildings. Fibre is non-conductive, so it provides galvanic isolation and eliminates the earth potential difference problem entirely. It is also immune to the interference from the mains feed and is not subject to the copper length limit.
Install a fibre cable in its own conduit in the existing trench, with a patch panel and media converters or fibre-capable switches at each end. If the conduit is too small or shared in a way that cannot be separated, a new trench or a different route may be needed.
If the client insists on copper, it would need surge protection at both ends and careful attention to earthing and bonding, but fibre is simpler, safer, and the better engineering answer for a 60 m inter-building link.
3. The client asks whether the existing earth bonding makes the copper option safe. How do you answer?
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Bonding the two earth systems reduces, but does not eliminate, the potential difference during a fault or a lightning event. Bonding is designed to keep touch voltages within safe limits for people and to give fault current a path, not to guarantee that two points on a copper data cable stay at the same potential during a surge.
A lightning strike or a large fault can raise the potential of one building's earth relative to the other by a large amount for a short time. The bonding conductor limits this, but the data cable and its connected equipment can still see damaging currents. That is why surge protection is used on copper links between buildings, and why fibre is preferred.
So the bonding is necessary and good, but it does not make a copper inter-building link equivalent to fibre.