Why do the lights dim when a big machine starts?

Why does a machine starting make the lights dim?

Because a motor is greedy for the first second or two. Before it is spinning it draws several times the current it uses once it is running, and current flowing through any resistance loses voltage on the way.

Every cable, every terminal and the supply itself has a little resistance. At normal load the loss is small and nobody notices. When a compressor, a saw, a lift or a large extractor starts, the surge multiplies that loss for a moment, and everything sharing the same route sees a lower voltage. Lights are where you notice it, because your eye picks up a flicker that a socket circuit never shows.

A short, shallow dip on a big motor start is normal. Lights that dip hard, stay dim, or dip when something small starts are telling you the route has more resistance than it should.

Where does the voltage get lost?

In one or more of four places, and the cure for each is different.

• The machine’s own circuit. A long run on a cable chosen for the running current, not the starting current, loses too much voltage at start.
• A shared submain. The commonest cause in a workshop or unit. The board the machine is on is fed by a cable sized years ago for less load, and the lighting is fed from the same board.
• A poor connection. A loose or corroded terminal behaves like a resistor. It also gets hot. Dimming together with a warm smell, discoloured accessories or a buzzing board is not a nuisance, it is a fault to get looked at this week.
• The incoming supply. Sometimes the dip is already there at the cut out, because the building is at the end of a long network cable or shares a transformer with heavy users.

BS 7671 sets voltage drop limits for an installation in normal use, a smaller allowance for lighting than for other circuits, and a good design checks the start condition of big motors too. An installation that has grown one machine at a time has usually never had that check.

How does an electrician find the cause?

By measuring, not guessing. We put a recorder on the supply at the cut out, at the board the machine is on and, where we can, at the machine itself, then start the machine and watch what the voltage does at each point.

If the dip is small at the cut out and large at the board, the submain is the problem. If it is small at the board and large at the machine, it is the machine circuit. If it is already outside the limits at the cut out, it is the network.

Alongside that we check the loop impedance of the circuits involved, look at what is actually connected against what each cable was sized for, and go over the terminations, often with a thermal camera while the load is on. A loose joint shows up as a hot spot before it shows up as anything worse. As the person in control of the premises you have a duty under the Electricity at Work Regulations to keep the system safe, and this is part of that.

Do we need a bigger supply, or three phase?

Rarely, and this is where a lot of money gets wasted. A new or upgraded supply makes more capacity available at the cut out. If the voltage is being lost inside the building, on a submain or a joint, a bigger supply feeds the same bottleneck and the lights dim exactly as before.

Three phase is the right answer when the real maximum demand is more than the supply can carry, or when a machine needs 400 volts. Our three phase article covers that decision. A dimming light is not on its own a reason to apply.

The exception is when the measurements show the voltage at your cut out falling outside the legal range. The supply is declared at 230 volts and must stay within 10 per cent above and 6 per cent below it. If it does not, that is the network operator’s to investigate, and we give you the recordings to send them.

What actually fixes it?

It depends on where the voltage is lost, which is why the measuring comes first.

• A new or larger submain to the board the machines are on, sized for the start condition rather than the running load.
• Separating the lighting onto its own board or a different route, so a motor start does not travel through the lights.
• Remaking poor connections and replacing damaged accessories found in the inspection.
• A soft starter or an inverter drive on the machine, which ramps it up instead of starting it in one hit. This is usually agreed with the machine supplier, because it affects how the machine runs.
• Rebalancing the load across the phases on a three phase board, where one phase is doing most of the work.

Often it is two of these together. The answer is in the readings, and we would rather show you them than sell you the biggest item on the list.

What changes the price?

We price from what the investigation finds, so the first job is the investigation itself.

• How many boards and machines need recording, and whether we can do it during normal working.
• The length and route of any new submain: through an occupied unit, along existing containment, or new.
• Whether the board the machines are on needs replacing or just a better feed.
• Out of hours working for the isolations.
• Any repairs the inspection finds along the way.

Tell us what dims, what starts and how long it has been happening through our quote page, or read how we handle commercial three phase and machine supplies.

Questions we get asked about this