Short-circuit protection is one of the most critical — and most misunderstood — aspects of electrical generator installation. Without it, a fault on the connected distribution system can cause catastrophic damage to the alternator windings, control panel, and connected equipment in a fraction of a second. With it correctly specified and coordinated, a fault is isolated quickly and cleanly, leaving the rest of the system intact and the generator ready to continue supplying power once the fault is cleared. This guide explains how short circuit protection works on a diesel generator, what devices are involved, and why getting the specification right matters as much as the generator's kVA rating.
What Is a Short Circuit in a Generator Context?
A short circuit occurs when an unintended low-resistance path is created between two live conductors, or between a live conductor and earth — bypassing the connected load entirely. In a generator context, this typically happens due to wiring insulation failure, a loose termination, equipment failure in the connected distribution system, or accidental contact between conductors during installation or maintenance.
The effect of a short circuit is an immediate, massive surge in current — potentially tens or hundreds of times the generator's rated output current — because the only impedance limiting current flow is the alternator's own internal impedance, which is low by design. Without protection, this current surge generates extreme heat in the alternator windings within milliseconds, causing insulation breakdown, winding damage, and in severe cases, fire.
Why Generator Short Circuit Current Is Lower Than the Grid
This is the point that catches many electrical designers and installers off guard. When a fault occurs on a mains grid supply, the available fault current at the point of fault can be very high — often several thousand amperes — because the grid draws on the combined generation capacity of the entire network. A diesel generator, by contrast, is a finite, self-contained source: its available fault current is limited by its own alternator impedance, typically expressed as the subtransient reactance (X"d).
In practical terms, a diesel generator typically delivers a peak short circuit current of approximately 3 to 10 times its rated full-load current for the first few cycles, before the AVR responds and the alternator's excitation reduces, rapidly dropping the fault current. This has two important implications: first, overcurrent protective devices (OCPDs) must be rated and set to operate reliably at these lower fault current levels, not at the high fault currents associated with grid supply; second, discrimination between protective devices at different levels of the distribution system becomes more challenging with a generator supply than with a stiff mains grid.
A protection scheme designed for a mains grid supply may not operate correctly when the building switches to generator power — particularly at the downstream end of long cable runs, where available fault current from a generator may be lower than the minimum operating current of an MCB or fuse. This must be verified at the design stage, not discovered during a fault event.
The Main Protective Devices Used on Generator Installations
Air Circuit Breakers (ACBs)
For larger generators — typically above 200 kVA — the main output circuit breaker is usually an Air Circuit Breaker (ACB). ACBs are designed for high-current applications (typically 800 A to 6,300 A) and include adjustable electronic trip units with configurable settings for overcurrent, short circuit, and earth fault protection. The key advantage of ACBs for generator applications is their adjustable tripping characteristics: instantaneous trip current thresholds, short-time delay settings, and long-time overcurrent curves can all be set to match the generator's fault current profile and coordinate with downstream protective devices.
Moulded Case Circuit Breakers (MCCBs)
For smaller to medium generators — roughly 50 kVA to 500 kVA — a Moulded Case Circuit Breaker (MCCB) is the typical main output protective device. MCCBs provide both overload and short circuit protection in a compact unit with higher interrupting capacity than standard MCBs. For generator applications, it is essential to verify that the MCCB's minimum instantaneous trip current (the level at which it trips without intentional time delay) is within the generator's available peak fault current — otherwise the MCCB may fail to trip on a short circuit, defeating its protective purpose entirely.
Miniature Circuit Breakers (MCBs)
MCBs protect individual final circuits in the connected distribution board. Their tripping characteristics — Types B, C, and D — define the minimum instantaneous trip current as a multiple of rated current (typically 3–5x for Type B, 5–10x for Type C, 10–20x for Type D). For generator supplies with lower available fault current, Type B MCBs are often specified for final circuits, as their lower instantaneous trip threshold means they are more likely to operate correctly on the reduced fault current a generator can deliver — particularly at the end of longer cable runs.
Overcurrent and Earth Fault Relays
Larger or more complex generator installations — particularly those supplying critical loads in data centres, hospitals, or industrial facilities — often include dedicated protection relays in addition to circuit breakers. These relays monitor current continuously and send a trip signal to the circuit breaker if a fault is detected. Inverse Definite Minimum Time (IDMT) relays are commonly used: they trip faster at higher fault currents and more slowly at lower overcurrents, providing both speed for serious faults and tolerance for transient motor starting currents that would cause nuisance tripping with an instantaneous trip device alone.
Protection Coordination and Discrimination
Coordination — also called discrimination or selectivity — means ensuring that when a fault occurs, only the protective device immediately upstream of the fault operates, leaving all other circuits unaffected. In a well-coordinated system, a fault on a single final circuit trips only that circuit's MCB, leaving the distribution board, the main circuit breaker, and the generator supply intact. A poorly coordinated system may see upstream devices tripping alongside the faulted circuit's device, causing a much wider loss of supply.
Achieving good discrimination on a generator supply is more challenging than on a mains grid supply, precisely because the generator's available fault current is lower and less stable. The protection engineer must verify that the time-current characteristics of devices at each level of the distribution system do not overlap — and that the generator's AVR response and fault current decay curve are factored into the coordination study. This is a design task that requires a qualified electrical engineer and cannot be assumed from standard mains-supply discrimination tables.
Earth Fault Protection: A Separate but Related Concern
Earth fault protection on a generator installation is governed by the earthing system configuration — TN-S, TT, or IT. The earthing arrangement on the generator must be considered independently from the mains supply earthing, since the two systems are electrically separate when the transfer switch is open.
For a generator operating as a standalone TN-S system, the neutral-earth link at the generator's output terminals creates the reference for earth fault current to flow. The automatic transfer switch (ATS) must switch both the live conductors and the neutral to ensure the generator's neutral-earth link is not in circuit simultaneously with the DNO's earthing arrangement — a condition that would create a parallel earth path with unpredictable fault current behaviour.
Residual Current Devices (RCDs) are often used as supplementary earth fault protection on generator supplies, particularly where the available fault current may be insufficient to guarantee reliable operation of overcurrent devices alone at the end of long cable runs. However, RCDs protect only against earth faults — they offer no protection against short circuits between live conductors, and overcurrent protection remains essential alongside any RCD.
Legal and Compliance Requirements
The Electricity at Work Regulations 1989 place a legal duty on operators and designers to ensure that electrical systems — including generator installations — are designed, installed, and maintained to prevent danger. Regulation 11 specifically requires that adequate protection against excess current is provided in electrical systems. Compliance with BS 7671 is widely accepted as the practical means of demonstrating compliance with the Electricity at Work Regulations, but the Regulations themselves are the legal instrument — following BS 7671 is not optional in practice.
For larger installations, it is advisable to have the protection coordination verified by an independent protection engineer before commissioning — particularly where the generator will supply safety-critical loads such as fire detection systems, emergency lighting, or medical equipment, where a protection failure during a fault event would have direct safety consequences.
Key Takeaways for UK Generator Buyers
- Never assume that a protection scheme designed for a mains grid supply will work correctly when the building is on generator power — always verify fault current levels at each protective device location.
- Specify MCCB and ACB trip settings to match the generator's actual fault current profile, not a generic table value.
- Use Type B MCBs on final circuits supplied from generators where cable runs are long or fault current may be low.
- Ensure the ATS design switches the neutral as well as live conductors, and that earthing arrangements are reviewed by a competent electrical engineer.
Verify protection coordination across the full distribution system from generator output to final circuit — this must be a design task, not an assumption.
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