Voltage transformer fuse-failure / loss-of-potential supervision (ANSI 60) — why a blown VT fuse can trip a healthy circuit

The guide on distance protection (21) and the guide on under/overvoltage protection (27/59) both rely on an accurate secondary voltage signal from a voltage transformer (VT). This article covers what happens when that signal disappears not because of a real system disturbance, but because of a failure entirely internal to the VT circuit itself — and the supervision function, ANSI device 60, built to catch it.

Why a lost VT signal looks like a fault

A VT's secondary output reaches the protection relay through a fuse or miniature circuit breaker and a length of wiring. If that fuse blows, that MCB trips, or a secondary connection comes loose, the relay's measured voltage collapses towards zero — while the actual primary voltage on the system, and the load current still flowing through the associated current transformers, remain completely normal. From the relay's point of view this looks deceptively similar to a genuine, severe fault: a sudden voltage collapse.

The consequence differs by protection function:

• A distance relay (21), which calculates apparent impedance from voltage divided by current, sees the impedance collapse toward zero as the voltage input disappears — exactly the signature of a close-in fault — and can trip on perfectly healthy load current.
• An undervoltage relay (27) can trip on the apparent loss of voltage.
• A directional relay (67/67N), which needs the voltage signal as a phase reference to determine fault direction, can lose its reference and misjudge direction entirely.

What fuse-failure / loss-of-potential supervision does

An ANSI 60 (or "VTS" / "60FL") scheme is designed to distinguish this specific failure mode from a real system fault, typically by comparing what the voltage and current signals are doing relative to each other:

• A real fault normally produces a voltage collapse accompanied by a simultaneous rise in current (and the appearance of negative- or zero-sequence current for an unbalanced fault).
• A VT fuse failure produces a voltage collapse — often characterised by the appearance of negative-sequence voltage — with no corresponding change in current: the load current stays exactly what it was before the voltage disappeared.

When the scheme recognises that voltage signature without the matching current signature, it declares a fuse-failure condition. Depending on the relay's configuration, this typically blocks the voltage-dependent protection elements (distance, directional, under/overvoltage) from tripping, while leaving voltage-independent backup protection (plain time-overcurrent) active, and raises an alarm so the actual VT circuit fault can be found and repaired.

Many installations complement the purely electrical detection with a simple auxiliary contact on the VT's MCB, wired directly into the relay as a fast, unambiguous "VT circuit open" input — useful because it detects the MCB tripping instantly, without needing to wait for the electrical signature to be recognised.

Note: the exact detection logic (which sequence-component voltage and current thresholds are used, and precisely which protection elements are blocked versus left active) is relay- and manufacturer-specific; this article covers the underlying principle, not one manufacturer's settings table.

Practical relevance

When commissioning or reviewing a distance or directional protection scheme, fuse-failure/loss-of-potential supervision should be verified as present and correctly configured — not assumed. A practical commissioning check is to remove the VT fuse (or open the VT MCB) under controlled conditions with load current flowing, and confirm that the scheme declares a fuse-failure alarm and blocks the voltage-dependent elements, rather than the relay tripping on what looks like a fault.

Common mistakes

1. Having no fuse-failure/loss-of-potential supervision at all on a distance or directional protection scheme — a simple blown VT fuse can then trip a completely healthy circuit.
2. Blocking the time-overcurrent backup protection along with the voltage-dependent elements during a declared fuse-failure condition — backup protection that does not depend on the lost voltage signal should generally remain active.
3. Relying only on the VT MCB's auxiliary contact, without any electrical detection of the voltage/current signature — a loose wire or a fuse failure that does not trip the MCB itself would then go undetected.
4. Never testing the fuse-failure scheme during commissioning — confirming it exists on paper is not the same as confirming it actually declares an alarm and blocks the intended elements when the VT signal is genuinely lost.

Related

• Distance protection (21) — impedance protection zones
• Under/overvoltage protection (27/59)
• Directional overcurrent protection (67)
• Synchro-check (25)