Busbar differential protection (ANSI 87B) — why a fault on the busbar itself needs its own, fast protection zone

The guide on transformer differential protection (87T) and the guide on REF protection (64N/87N) both cover the differential principle applied to a transformer winding: compare what goes in with what comes out, and an imbalance indicates an internal fault. This article covers the same principle, but applied to a fundamentally different part of the installation: the busbar of a switchboard itself, protected by busbar differential protection, in practice referred to by the ANSI code 87B.

Why a fault on the busbar is a separate problem

A busbar typically connects several infeeds (for example two transformers) to several outgoing feeders. A fault occurring on the busbar itself — for example due to a failed bus insulator, a forgotten tool or contamination leading to flashover — is not automatically cleared quickly by the ordinary infeed or outgoing-feeder protection: those are primarily set to clear faults in their own zone (the cable or the transformer) quickly, and often only see a busbar fault via their slower, further upstream back-up setting. Because a busbar typically forms the heart of a switchboard — with a very high available short-circuit current, fed from multiple directions at once — a slowly cleared busbar fault means a significantly higher energy input into the fault than a fault further along a single outgoing feeder, with a correspondingly increased risk of arc-flash damage (see the [guide on arc flash](/guides/nen-3140/vlamboog)) and damage to the busbar itself.

The differential principle applied to a busbar zone

87B compares, just like 87T, the sum of all currents entering and leaving the protected zone — but instead of the two (or three) windings of a single transformer, this is now all connections to the busbar: every infeed and every outgoing feeder gets its own current transformer (CT), and the vector sum of all these currents is continuously monitored:

In a healthy state, this sum is close to zero: all current entering the busbar via the infeeds leaves the busbar again via the outgoing feeders.
With a fault inside the busbar zone, an imbalance arises — current enters the zone that does not leave again via one of the monitored connections (the fault itself is the "leak") — and the relay trips.
With a fault outside the busbar zone, for example further along an outgoing cable, the sum of the busbar CTs stays balanced (the fault current simply leaves via that outgoing feeder and is counted there as well), and 87B does not trip — that fault is cleared by the outgoing feeder's own protection.

High-impedance versus low-impedance: the same trade-off as with REF

Just as with REF protection, two common implementations of 87B exist:

High-impedance: all CTs are connected in parallel to a relay with a stabilizing resistor in series, so that CT saturation during a heavy external fault does not cause unwanted tripping. This requires, just as with REF, a sufficiently high knee-point voltage of every CT involved (see the guide on CT protection class and knee-point voltage) relative to the relay setting.
Low-impedance: a numerical relay digitally computes the differential current from the individual CT measurements, with a percentage-biased restraint characteristic comparable to an 87T relay. This makes sharing CTs with other protection functions easier, but sets higher requirements for matching the CT saturation characteristics across all connected feeders.

Why the zone boundaries of 87B must be precisely correct

Unlike a single winding with typically two or three CTs, 87B sums the currents of all connections to the busbar — sometimes dozens of CTs at once for a busbar with many feeders. If a feeder is accidentally not included in the zone (for example after a switchboard extension that was not fully carried through in the 87B configuration), a structural imbalance arises that either causes the relay to trip unnecessarily under normal load current, or — if the sensitivity is set lower to avoid this — causes it to miss a genuine busbar fault instead.

Note: the exact busbar topology (single bus, double bus with bus-coupler, ring bus) determines how the protection zones are exactly delineated, particularly around the bus-coupler between two bus sections; this article covers the principle, not a ready-made zone layout for every busbar configuration.

Practical relevance

When extending or modifying a switchboard with busbar differential protection, it must always be checked that the new or modified feeder is actually included in the 87B zone configuration — a busbar extension that has only been carried through mechanically and in the feeder protection, but not in the 87B CT wiring, undermines the stability and selectivity of this protection.

Common mistakes

Assuming the ordinary infeed or outgoing-feeder protection clears a busbar fault just as fast as a fault in its own zone — without 87B, a busbar fault is often only picked up via a slower back-up setting.
Not including a new or modified feeder in the 87B zone configuration during a switchboard extension — this causes a structural imbalance or a blind spot in the protection zone.
Not reassessing the knee-point voltage of busbar CTs after replacement in a high-impedance 87B scheme — just as with REF, a CT with too low a knee-point voltage undermines stability during a heavy external fault.
Ignoring the bus-coupler between two bus sections when determining the zone boundaries — the position of this switch determines whether both bus sections should be treated as one or as two separate 87B zones.