Arc time factor vs duty cycle — two numbers people keep confusing

Arc time factor is the share of available working time during which an arc is actually burning. Duty cycle is a rating printed on the power source describing how long it can deliver a given current before it must cool. One is about how a shift is spent; the other is about what the equipment can survive. They are not interchangeable, and treating them as though they are is a reliable way to produce a capacity plan that overstates output by a factor of three.

Arc time factor

Also called operating factor or arc-on time, arc time factor is:

arc time factor = arc-on time ÷ available time × 100%

The subtlety is entirely in the denominator. Available time can mean paid hours, attended hours at the station, or scheduled production time — and the answer changes dramatically depending on which you pick. A cell measured against attended time will look far better than the same cell measured against paid hours, because breaks, meetings and travel land outside the numerator either way.

So the number is meaningless unless the denominator is stated. If you take one thing from this article, take that. Before comparing two sites, two shifts or two vendors' claims, establish what each is dividing by.

Everything that is not arc-on time is the interesting part: fit-up, tacking, positioning, changing consumables, grinding, cleaning, inspection, waiting for a crane, waiting for a permit, waiting for the part. Manual welding spends most of the shift there. Commonly reported arc time factors for manual processes sit somewhere in the region of 10–30%, with mechanised and robotic work considerably higher — but treat any published benchmark with suspicion until you know its denominator and whether it came from measurement or from someone's estimate.

Raising arc time factor is usually not a welding problem. It is a material-flow, fixturing and scheduling problem, and it is why the metric is worth tracking: it points at the hours you are paying for and not welding.

Duty cycle

Duty cycle is an equipment rating defined by IEC 60974-1. It is expressed as a percentage at a stated current over a 10-minute reference period, at a stated ambient temperature.

A machine rated 60% at 400 A can deliver 400 A for six minutes out of every ten, then needs the remaining four to cool. Exceed it and thermal protection intervenes — or, over time, the machine's life shortens.

Three things people get wrong:

• The period is ten minutes, not an hour or a shift. Duty cycle says nothing about how a day is spent.
• It is current-dependent. The same power source will show a much higher duty cycle at lower current. A quoted duty cycle without its current is incomplete.
• It is rated at a reference ambient temperature. Hot environments derate it, which is a real constraint in summer or in enclosed bays.

Side by side

The mistake this causes

The failure is always the same shape. Someone reads 60% duty cycle on a datasheet and plans capacity as though the station will weld for 60% of the shift. The real arc time factor turns out to be 20%. The plan overstates output threefold, and the shortfall gets blamed on the welders.

It runs the other way too. A station showing a low arc time factor sometimes gets "fixed" by buying a higher-duty-cycle machine — which changes nothing, because the machine was never the constraint. The welder was waiting for parts.

Measuring it honestly

Self-reported arc time is unreliable in a specific direction: it is estimated at the end of a shift, from memory, by someone who has no reason to be pessimistic. It also tends to count attempted welding rather than arc-on time.

To get a number you can plan against:

1. Take arc-on time from the power source, not from a timesheet. The machine knows precisely when the arc was live.
2. Fix the denominator and write it down. Attended time is usually the most actionable, because it isolates what happens at the station from how shifts are scheduled.
3. Segment before you aggregate. A site average hides everything. Per station, per shift and per job is where the recoverable hours show up.
4. Establish a baseline before changing anything, then compare the same period after. Without a before, an improvement is an assertion.

Once it is measured rather than estimated, arc time factor becomes the input to the numbers that matter commercially — cost per weld hour, cost per metre, and whether a quote reflects what the shop actually does.

IEC 60974-1 is the standard governing duty cycle ratings for arc welding power sources. Arc time factor has no equivalent standard definition, which is exactly why stating your denominator matters.