Site Power Keeps Dropping Out: What to Check

Site power problems cost a day's work before anybody even arrives. Most of them are one of six causes, and most of those you can identify yourself in ten minutes standing at the board.

Worth saying up front: a temporary supply is governed by its own standard and requires periodic testing. If yours has been on site for months and nobody has tested the RCDs, start there rather than here.

First: is it your supply or the street?

Look at the neighbours and at any street lighting. If the whole street is out, it is the distributor's and nothing on your board will help.

If it is only your site, work down the board from the top: is the main switch on, is the RCD on, are the individual outlet protective devices on. Whichever device has operated tells you where to look.

Cause 1: RCD tripping, and it trips again straight away

The most common site call, by a wide margin.

An RCD trips on earth leakage. On a construction site there is a great deal of legitimate leakage available: water, damp concrete, long leads and motors with damp windings. There is also a lot of illegitimate leakage from damaged leads.

The ten minute diagnosis. Unplug everything. Every lead, every tool, every light. Reset the RCD.

• It holds with everything unplugged. The board is fine and one of the things you unplugged is the problem. Plug them back in one at a time, running each briefly, until it trips. That item is your fault. Nine times out of ten it is a lead, and usually a visibly damaged one.
• It trips immediately with everything unplugged. The fault is in the installation or the board itself. That is a call-out.

The one to look for specifically: a lead lying in water, or a lead that has been driven over. Both produce exactly this symptom, and taping a damaged lead does not fix it.

Cause 2: RCD trips, but only sometimes

Harder, and usually environmental.

It will often correlate with rain, with early morning damp, or with one particular piece of plant starting. Motors draw hard at startup and a motor with damp or aged windings leaks most in that first moment.

What to do: note what was running each time it happens. Three data points usually identify it. Intermittent leakage that correlates with weather is nearly always a lead, a connector sitting on the ground, or a tool that needs testing.

Cause 3: the main switch trips, not the RCD

Different problem entirely, and worth telling apart because the fix is different.

A main switch or circuit breaker trips on overload, not leakage. It means the site is drawing more than the supply is rated for.

What to do: count what was running. A saw, a compressor, a mixer and a kettle on a 32 amp single phase supply will do it. If it only happens when several trades are going at once, the supply is undersized for how the site is actually being used, not faulty.

The fix is either sequencing the work, or upsizing the supply. Do not fit a larger device to stop it; the device is sized for the cable and the connection behind it.

Cause 4: no power at one outlet, others fine

Usually the outlet, its protective device, or its termination.

What to check: whether that outlet has its own device on the board and whether it has operated. Then whether anything heavy has been plugged into it, because a 10 amp outlet with a 15 amp plant load through an adaptor is a warm outlet becoming a dead one.

A warm or discoloured outlet is a loose contact, and that is a call-out rather than a reset.

Cause 5: tools running badly rather than not at all

No tripping, but the saw sounds slow, the compressor labours, or the lights dim when something starts.

This is voltage drop, and on sites it is nearly always the leads. Long leads, thin leads, and leads daisy-chained end to end all drop voltage, and a tool running at low voltage draws more current to do the same work, which makes it hotter and makes the drop worse.

What to do: shorten the run, use a heavier lead, and stop chaining them. If the site has grown away from the board, a sub-board closer to the work beats 60 metres of extension lead.

If the tools run badly with a short heavy lead straight into the board, then it is the supply rather than the leads, and that is worth a call.

Cause 6: nothing at all, and nothing has tripped

Dead board, everything in the on position.

That is upstream: the incoming supply, the connection, the metering or the main switch itself. Nothing on the board will fix it and nothing should be opened to look.

What to do: check whether the street has power, then call. This one is connection-side work and it needs an ASP.

What not to do on a site board

Do not fit a larger fuse or breaker. Covered above, and it is the single most dangerous thing on this page.

Do not bypass an RCD. It happens, on sites where nuisance tripping has become a productivity problem, and it removes the only protection standing between a person and a damaged lead.

Do not tape a damaged lead. Tape restores the appearance of insulation and none of its function.

Do not open the enclosure. Metering and incoming supply are inside it. Locked is how it should be.

Do not run a lead from a neighbouring property. No protection arrangement, no metering, and a lead doing a job it is not rated for.

The maintenance that prevents most of this

Four things, and they take ten minutes a week:

1. Press the RCD test buttons. They should trip instantly. Reset them. Record it. This is a requirement on a temporary installation, not a nice-to-have, and it is the one that gets skipped.
2. Walk the leads. Look for crushing, cuts, exposed conductor and taped repairs. Replace rather than repair.
3. Get leads off the ground where water sits. Hooks, stands, anything.
4. Keep the board accessible. As a build progresses, boards end up behind stacked materials and inside scaffold. Somebody needs to reach the main switch in a hurry.

Frequently asked questions

Is nuisance tripping just something you live with on site? No. Repeated tripping has a cause, and on most sites it is one identifiable lead or tool. Treating it as unavoidable is how boards end up with the RCD bypassed.

Can I reset the RCD as many times as I need to? If it holds, the leakage was transient and you have your answer from what was running. If it trips again immediately, resetting it repeatedly is overriding a decision it is making about a real fault.

My supply was fine for six months and now it trips daily. What changed? Usually the weather, or the site. Damp gets into connectors that were fine in summer, and as a build progresses leads get longer, more numerous and more damaged. It can also be an RCD itself deteriorating, which is what the periodic testing is for.

Do I need an electrician or an ASP? Anything on the board's outgoing side, including outlets and leads, is ordinary licensed electrician work. The incoming supply, the metering and the connection are ASP work. If the board is completely dead with nothing tripped, that is the ASP end.

Can the supply be upsized mid-build? Yes, though it means an application and an outage. If the site is tripping the main switch regularly, it is usually cheaper than losing time.

Who is responsible for testing while it is on site? The person conducting the business or undertaking on the site. On an owner-builder job, that is the owner. The ASP who installed it is not coming back unless asked.

If it is cause 6, or the board itself

Call 0485 824 714 and describe which device operated and what was running. That distinction between the RCD and the main switch tells us most of what we need before we get there.

We are accredited Level 2 ASPs under the NSW Government's ASP Scheme, authorised to work on the distribution network, with 15 years on connection-side work and a Lifetime Labour Warranty on what we do. Available 24 hours a day, free quotes, across Sydney and the Blue Mountains on both the Ausgrid and Endeavour Energy networks.