When a planned outage takes a mine’s normal high-voltage supply offline, production doesn’t simply stop. Ventilation fans, dewatering pumps, conveyors and critical services still have to run — so operators turn to temporary generator plant to carry the site through.
Hiring the generators is the easy part. Making sure that temporary supply can actually power a large, complex mining reticulation network — safely, reliably, and with protection that still works — is where the real engineering lives. And it’s a part of the job that often falls through the cracks.
The generator supplier engineers the farm — not your network
Generator hire companies are very good at what they do: sizing, delivering and commissioning a generator farm. What they don’t do — and readily admit sits outside their scope — is model the site they’re plugging into.
That’s a problem, because a mine’s electrical network is nothing like the generator farm’s terminals. Long cable and aerial runs, step-up and distribution transformers, deep underground workings, large motors, and existing protection schemes all sit between those generators and the equipment that has to keep running. Connecting a temporary source into that network changes the behaviour of the entire system.
Without a study of the whole picture, you’re relying on assumptions. And in a planned outage, assumptions are expensive.
Why generators change everything electrically
A utility supply delivers enormous fault current. A generator farm delivers a fraction of it. That single difference cascades through the design:
Fault levels collapse. Protection relays across the network were set for grid-level fault current. On generators, the available fault current can drop to a fraction of that — and relays that were perfectly graded on utility supply can become dangerously insensitive, potentially never seeing a genuine fault.
Motor starting and inrush bite harder. Starting large motors and energising transformers on a low-fault-level source causes deeper voltage dips and instability — enough to stall plant or trip the generators entirely if it isn’t modelled and staged.
Sizing is a real risk, both ways. Undersize the plant and you shed load mid-outage. Oversize it and you burn budget on generators you never needed. Only a load flow across realistic operating scenarios tells you what you actually require.
What a proper temporary-power study delivers
This is where an independent, site-side power system study earns its keep. Using an industry-standard modelling package such as SKM PowerTools, the site’s network model is updated with the temporary generator setup and run through every credible scenario:
- Load flow and generator sizing — confirming the plant carries the site under real, diversified operating conditions, including single-unit-down contingencies.
- Fault level (short-circuit) study — quantifying how far fault current collapses at every point in the network, at each number of running generators.
- Protection review — checking every relay is still sensitive enough to detect and clear a fault at the reduced fault level, and recommending corrected settings where it isn’t.
- Motor starting and inrush analysis — proving large starts and transformer energisation won’t stall plant or destabilise the generators.
- Clear operating limits — a plain-English report the site and the generator supplier can execute against, including how many generators must be running to operate safely.
The deliverable isn’t a stack of numbers. It’s confidence that the outage plan will work before the site commits to it — and a set of settings and rules the crews can actually follow on the day.
Proven on real mine outages
JMB Engineering has delivered exactly this on live underground coal operations. On one recent planned outage, two temporary generator farms powered a mine’s surface, development and longwall equipment through the 11 kV network. Our study found the network’s fault levels collapsed so far that the supplier’s recommended protection settings would have left the site under-protected. We specified corrected, standards-compliant settings that stayed sensitive at the reduced fault level, and translated the fault study into a simple operating rule — how many generators must run for the site to stay safe.
That combination — power system modelling depth plus practical mine-site experience — is what turns a risky temporary supply into a de-risked, executable plan.
Planning an outage? Get the engineering right first
As one of Australia’s leading power system modelling specialists in mining, JMB Engineering models the integration, proves the site can run safely, and gets the protection right — whether or not we already hold your network model.
If you’re planning a shutdown that relies on temporary generator power, talk to us before the plant is finalised. We’ll prove it works first.
📄 Download our Temporary Power capability sheet
Get in touch: admin@jmbengineering.com.au · 0420 966 776 · jmbengineering.com.au
