PEN conductor break in TN-C-S — the hidden danger

On a large agricultural operation, virtually all metalwork — greenhouse frame, piping, distribution boards — is bonded together and to PE (see the guide on main bonding). That is precisely why a specific, little-known failure mechanism has extra consequences here: a break in the combined PEN conductor of a TN-C-S system.

How it goes wrong

In the TN-C section of a TN-C-S system (before the point of separation into separate N and PE conductors), neutral and earth are combined into a single PEN conductor. If that PEN conductor is interrupted — a terminal that has vibrated loose, corrosion, mechanical damage — the following happens:

1. The neutral conductor starts to "float": without a fixed connection to the star point of the transformer, the voltage of the neutral conductor, via the connected loads, takes on a value that can drift toward the phase voltage.
2. Because PE downstream of the separation point is connected to that same (now floating) neutral conductor, that elevated voltage also appears on all earthed metal parts downstream of the break — in the worst case, nearly the full phase voltage on a greenhouse frame, pipe, or enclosure that is normally safely earthed.

Why an RCD does not reliably catch this

A common assumption is: "there's a 30 mA RCD everywhere, so I'm protected against any earthing problem." A PEN break is a concrete counterexample here: the danger arises from a floating, elevated potential, not necessarily from a leakage current to true earth that an RCD would detect as a difference between phase and neutral current. An RCD can therefore let pass unnoticed something that does pose a lethal danger on touched metalwork.

Practical measures

• The PEN conductor must never be switched or fused — a fuse or switch in the PEN conductor could allow a deliberate switching moment to coincide with exactly this dangerous failure mechanism.
• Mechanical protection and generous cross-section of the PEN conductor, precisely because a break here has more serious consequences than a break in a regular N or PE conductor separately.
• Visual inspection of the separation point (the transition from TN-C to TN-S, usually at the main distribution board) for corrosion and terminal tightness — belongs in the periodic NEN 3140 inspection, with extra attention on agricultural operations due to moisture and corrosive ambient air (manure, fertilizers).

Common mistakes

1. Assuming a 30 mA RCD provides protection against every conceivable earthing problem — a PEN break is one of the scenarios where that does not necessarily hold true.
2. Treating the PEN conductor as an ordinary cable in terms of cross-section selection or mechanical protection, without weighing the elevated consequential risk of a break in it.
3. Not explicitly including the TN-C→TN-S separation point in the periodic inspection round, while this exact point is vulnerable to corrosion in a damp agricultural environment.

Related

• Main bonding for greenhouse structures — §544
• Residual current devices (RCD) — §531
• Earthing — §411