What Load Shedding Does to Your Refrigeration Equipment

Every stage of load shedding ends the same way for a cold room or condensing unit: the power comes back, the compressor restarts, and it looks like nothing happened. What actually happens electrically during that restart — voltage sag, high inrush current, a compressor starting against pressure it hasn’t had time to equalise, contactor contacts absorbing an arc they weren’t sized for — is invisible unless you measure it, and it is the reason equipment on load-shedding sites fails years earlier than the same equipment on stable supply. By the end of this article you’ll understand exactly what the restart cycle does to a compressor and its contactor, and which three protections actually stop the damage rather than just delaying it.

What Happens Electrically the Moment Power Returns

When supply is restored after an outage, every compressor on that feeder — not just yours — restarts at the same instant. Two things happen together, and they compound each other.

Every compressor draws inrush current during restoration, typically five to seven times its full-load current, exactly as it would on any cold start. On a long feeder carrying a whole neighbourhood’s simultaneous restart, the combined demand produces a deep voltage sag — often 180 V to 200 V for several seconds on a nominal 230 V supply, well below the statutory ±10% tolerance band of 207–253 V.

That sag matters because motor starting torque falls with the square of applied voltage, not linearly. A supply sagging to 200 V (87% of rated) delivers only about 76% of rated starting torque; at 180 V (78% of rated) torque falls to around 61%. Every compressor on that feeder is therefore trying to start on a fraction of its rated torque, at the exact moment it also needs maximum torque to break away against head pressure. Many simply fail to start, trip on locked rotor, and sit waiting for the next reset.

Repeated Restarts and Short Cycling

A compressor design figure of around six starts per hour assumes normal thermostat cycling with time to cool down between starts. Load shedding does not respect that figure. A site running four stages of load shedding a day, each with a trip-and-reset cycle before the compressor settles into a stable run, can push a compressor to fourteen starts an hour or more — more than double its design limit.

Every one of those starts passes locked-rotor current — commonly on the order of 140 A on a 24 A compressor, in line with the 5–7 times FLA figure above — through the windings and the contactor’s contact tips. Windings sized for occasional starting current absorb thermal soaks faster than they dissipate the heat between cycles, shortening insulation life before any single event looks like a failure. The contactor pays a more visible price: repeated arcing at each make and break erodes the silver contact surface, raising contact resistance, which raises heat at that pole, which accelerates the erosion further — a wear cycle covered in how to test a contactor properly, including the voltage-drop measurement that catches a contactor already on this path before it welds shut or burns a pole open.

Starting Against Head Pressure

A scroll or reciprocating compressor that stops abruptly has full discharge pressure on one side of the system and suction pressure on the other. That differential needs time to bleed away and equalise before the next start — typically 3 to 5 minutes. Restore power inside that window and the motor is starting against a pressure differential it was never designed to break away against: effectively a locked-rotor start regardless of how healthy the motor is.

Combine that with the voltage sag above — reduced torque exactly when maximum torque is needed — and a compressor that would start without complaint on a stable supply, with equalised pressures, stalls repeatedly during a load-shedding evening. Each stall is another full locked-rotor current event through windings and contacts that have already absorbed several that hour.

Inverter Equipment: A Different Kind of Stress

Fixed-speed compressors aren’t the only casualties. Inverter-driven compressors and fans carry a DC bus — a bank of electrolytic capacitors storing the rectified supply voltage — and every restart charges that bus from zero, drawing a substantial inrush current through the rectifier into the capacitor bank. Where load shedding forces several outages and restarts in a single day, that charge-discharge cycle repeats every time.

Electrolytic capacitors have a finite number of charge cycles, and their service life shortens with heat and repeated stress — the same restart pattern that erodes a contactor’s contacts on fixed-speed equipment accelerates capacitor ageing on inverter equipment instead. A restart delay of 3 to 5 minutes after supply returns reduces this stress the same way it protects a fixed-speed compressor, and confirming this delay is fitted and correctly set on inverter equipment is worth checking specifically, not assumed because the equipment is newer.

The Practical Protections

Three measures address the mechanisms above directly, and none of them are expensive relative to a replaced compressor or a burnt contactor.

The restart delay is doing the most work of the three: it addresses both the pressure-equalisation problem and, by staggering when each unit reconnects, reduces how deep the feeder-wide voltage sag gets in the first place, since not every compressor on the line is inrushing at the same second.

Why a Poorly Regulated Generator Can Be Worse Than No Power at All

A generator brought in to bridge load-shedding outages solves the immediate problem — the plant keeps running — but poor voltage regulation can put equipment through worse stress than simply waiting for utility power. Utility supply, even sagging under restoration load, stays reasonably bounded near nominal; an undersized or poorly regulated generator can swing well outside the ±10% tolerance band in both directions as its load changes — under-voltage starting a large compressor, over-voltage once that load drops off. Both mechanisms above (torque loss on under-voltage, capacitor and insulation stress on over-voltage) apply to a generator exactly as they do to the grid, more often and more severely.

Before commissioning any generator changeover, confirm phase rotation on the generator supply specifically — it is not guaranteed to match utility rotation, and a reversed generator runs every scroll compressor on site backward, with no obvious symptom beyond a loud rattle and a compressor that develops no pressure differential at all. Check it with a phase-sequence meter before you rely on generator power to run refrigeration overnight.

Safety note: any work around a generator changeover involves confirming the changeover switch genuinely isolates the utility side before the generator is live, and applying earths wherever a generator or inverter could back-feed, per your isolation procedure under the OHS Act 85 of 1993 and SANS 10142-1. Never assume a changeover switch is working correctly without proving it.

For Business Owners: What This Actually Costs

If you run a cold room or a bank of condensing units through regular load shedding without these protections, the cost doesn’t show up as a single event — it shows up as compressors and contactors failing years ahead of schedule, on equipment that “was working fine” the week before. A replaced compressor and callout costs considerably more than a phase-failure relay and restart timer fitted once, and the protections work every load-shedding cycle rather than once. If a site has replaced the same compressor more than once in a year, the restart cycle itself — not bad luck with parts — is very often the cause, and it’s worth checking for voltage imbalance on the three-phase supply at the same time, since a marginal imbalance a compressor tolerates on a stable supply becomes a much bigger problem stacked on repeated restoration sags.

Final Thoughts

Load shedding doesn’t damage refrigeration equipment through any single dramatic event — it damages it through repetition: inrush current at five to seven times FLA on every restart, voltage sag during restoration cutting starting torque to roughly 61–76% of rated, compressors starting against pressure that hasn’t had 3 to 5 minutes to equalise, and contactor contacts absorbing an arc every single time. A phase-failure relay, a properly set restart delay, and surge protection at the panel address these mechanisms directly and cost a fraction of what a burnt-out compressor or welded contactor costs to replace. The full electrical picture behind these numbers — and how to verify a site’s specific readings against them — is in our guide to electrical troubleshooting and testing.