Generator quotes often list an ATS or a paralleling panel as if everyone knows what they are. This guide is for the facility manager, project engineer or contractor who has to decide whether an automatic transfer switch, generator paralleling or both belong on the site. Each part ends with questions for the supplier and the electrician.
What does an automatic transfer switch (ATS) do?
An automatic transfer switch moves your load between two supplies: the mains and the generator. It does four things in order.
- It senses that the mains supply has failed.
- It signals the generator to start.
- When the generator is ready, it moves the load onto it.
- When mains returns, it moves the load back and lets the generator stop.
The most important job is the one you never see. The generator and the mains must never feed the same circuit at once, unless the arrangement is purpose-built and interlocked. A generator wrongly connected can send power back into the grid. That can endanger anyone working on the lines.
Watch out. Never plug a generator into a building or site distribution board with a makeshift lead. Use a proper changeover arrangement installed by a qualified electrician.
When does a site need an ATS and when is a manual changeover enough?
Use one test: must the generator take over without anyone being there to switch it? If yes, you need an ATS. If someone is always on site and a short wait is acceptable, a manual changeover can be enough.
| Situation | Usually suits | Why |
|---|---|---|
| Unattended site or building at night | ATS | Nobody is there to switch. |
| Cold store, server room, medical or safety loads | ATS | An outage causes loss or risk. |
| Site office or camp with staff on hand | Manual changeover or ATS | A short wait may be acceptable. |
| Temporary power for a tool or short job | Often neither | The generator is the only supply. |
If the generator is the only supply, as on many early construction sites, there is nothing to transfer between. In that case you need correct connection and protection, not an ATS. Automatic also means unattended: the set needs enough fuel (see generator fuel planning) and a charged starting battery when the mains fails, so agree who checks them. Work out which case you are in before you rent anything. The guide on finding out industrial site equipment requirements shows who to ask.
What should I check on the ATS: rating, poles and transfer type?
Ask the supplier for these five things in writing.
- Current rating in amps. The ATS must be rated for the current your load draws. A kVA figure for the generator does not tell you the switch rating. Also ask for its short-circuit withstand rating, and let the electrician confirm it suits the site. To see how kVA, kW and current connect, read how to size a generator.
- Phases and poles. The ATS must match your supply: single or three phase, and the right number of poles. Ask whether the neutral is switched. Your electrician will know why it matters for your installation.
- Voltage. It must match the site supply.
- Auto-start on the generator. The ATS can only start a set that is built to start on its signal. Ask whether the set has the controls for this and who connects the two.
- Transfer type. Open transition means a brief break in power while the load moves over. Closed transition means no break, but it needs synchronising equipment and may need the approval of the network operator. Confirm with the electrician.
Even with an ATS, power is off after a mains failure while the generator starts and settles. Many backup uses accept a short break. Computers, process control and other sensitive loads may not. If you have such loads, tell the supplier and ask the electrician how to protect them.
What is generator paralleling?
Paralleling means running two or more generators together on a common bus, so they share the load between them.
The sets must be synchronised before they connect. Voltage, frequency and phase must all match. A paralleling controller does this automatically. Closing two unsynchronised sets onto one bus can cause serious damage to the sets and the installation. That is why sets are never paralleled by simply joining their output cables.
Why parallel two sets instead of renting one big set?
Use a second test: is one set too small, or do you need a spare if one fails? If either is true, consider paralleling.
| Reason | What paralleling gives you |
|---|---|
| One set is too small | Two or more sets add up to the demand. |
| Demand changes through the project | Sets can be added or removed as the load changes. |
| Outage is not acceptable | If one set stops or needs service, the others can carry a reduced load. |
Redundancy has a limit. If one of two equal sets fails, the other carries only its own capacity. Decide in advance which loads must stay on, and agree that with your electrician.
A single large set can still be the better choice for a steady, simple load. Paralleling adds panels and controls, which means more equipment to commission and more that can fail. Compare the options on what the site needs. For how ratings affect this, see prime, standby and continuous generator ratings.
What is needed to parallel generators safely?
Paralleling is a system, not just two machines. It needs three things.
- Compatible sets. Sets of the same type and make are easiest. Mixed sets can sometimes be paralleled, but only if the supplier confirms it.
- A paralleling panel and control scheme. This is where the synchronising and load sharing happen.
- A qualified electrician. Connecting generators or an ATS to a building or site distribution is their work.
Questions for the supplier:
- Can the sets you offer be paralleled with each other? Confirm in writing.
- Do the sets come with the paralleling panel and controller, or must we supply them?
- What happens to the load if one set trips?
- Who commissions and tests the system on site?
Requirements differ by project, site owner and authority. Confirm them before mobilising.
Where do transformers fit in?
Sometimes the generator voltage and the site voltage do not match. A transformer can bridge them.
Ask the electrician early whether your site needs one. Then ask the supplier whether a transformer of the right rating can be supplied with the generators. Do not leave it until the sets arrive.
How to do this on Maedah
Browse generators as a guest to see specs and cities. For the rental basics, see how to rent a generator for a construction site. Or post one request with the number of sets, the city and site on the map, and the dates.
In the requirements, say whether you need an ATS, or paralleling, or both. Add the ATS rating you need in amps, the phases and the voltage. Suppliers who offer that category and serve your city send sealed quotes. Compare them side by side: what is included, response time and rating. Use the chat on each quote to ask the questions above. Agree price and payment with the supplier.
