Manual or automatic transfer switch for your generator?

A manual switch is enough when someone is always close by and a few minutes without power do no harm. Choose an automatic transfer switch when nobody should have to walk to the generator at night. If your servers, controls or lines cannot even survive 10 seconds, put a battery in front of the generator. Our guide to the right emergency power supply for your business compares UPS, generator and battery in general. This article is about the switch between your grid, your generator and your loads. A battery that only waits for a cut rarely pays. Business battery systems that also cut your peak and store solar power do.

Maybe you bought the generator after one bad cut. Or it came with the building. It stands in the yard, and the plan is simple: when the power goes, someone walks out and throws the switch. Then the cut comes at three in the morning, on a Sunday. Or someone does throw the switch, but the servers and the line have already stopped.

Here is what is easy to miss. A generator is only as good as the way it gets connected to your loads. A manual switch depends on a person. An automatic switch still leaves a gap of seconds, and every motor restarts at once. A generator that has not run under load for a year may not start at all.

A customer in Maasmechelen had a generator for emergencies, just waiting. Our semi-off-grid system made it genuinely useful. The customer can now choose between the grid and the generator as the charger for his own off-grid system, with 24 kVA and 30.8 kWh.

Can you connect a generator without a transfer switch?

No. On a site with a grid connection, the generator must never be able to meet the grid. A transfer switch makes sure your loads hang on one source at a time: the grid or the generator.

Without it, the generator could push power back into the street cable, while grid technicians expect that cable to be dead. The switch also stops the generator from trying to power the whole street and overloading.

The Belgian electrical code AREI (in French RGIE) is clear on the device itself. The switch that changes between two sources must act on all live conductors, so the neutral too. And it must never put the sources in parallel by accident. In a TN system the earthed conductor may stay closed, if both sources use the same earth connection.

The grid rules follow the same line. Synergrid C10/11, the Belgian rule for generation units on the grid, does not apply to a back-up system that cannot deliver power towards the grid. Such a system only feeds the loads behind it. Our article on what a company should do about a blackout in Belgium explains why back-up is worth thinking about now.

Manual or automatic transfer switch: what is the difference?

A manual switch is moved by a person. An automatic transfer switch (ATS) watches the grid, starts the generator and switches by itself. Both leave a gap while the generator starts. Only a battery system closes that gap.

A manual switch usually has three positions: grid, OFF and generator. You always pass through OFF, so the two sources never touch. For a 3-phase site with a neutral, that is a 4-pole switch.

An open transition switch is "break before make": it opens one source before it closes the other. That is the usual type for emergency systems. A closed transition switch is "make before break": it connects the generator to the grid for a moment, which needs both sources in sync.

How does an automatic transfer switch start the generator and switch over?

The ATS senses that the grid is gone, gives the generator a start signal, and waits until its voltage is stable. Only then does it connect your loads. When the grid returns and stays, it switches back and lets the generator cool down without load.

How long does that take? The American emergency power standard NFPA 110 has a "Type 10" class. It requires acceptable power within 10 seconds of an outage. That is the benchmark for good standby generators. Ten seconds is long enough to stop servers, controllers and production lines.

The start itself is a heavy moment too. Electric motors draw two to twelve times their running current for a few seconds, according to Auburn University. When the ATS connects the whole site at once, every compressor, pump and fan wants to start together. The generator must be sized for that moment, not only for the running load.

An ATS can only start a generator that has a remote start input and an automatic engine controller. Our article on a generator with a battery explains how that two-wire start works in an off-grid system.

When is a manual switch enough, and when must it be automatic?

For safety installations the AREI decides for you: the safety source must switch in automatically. Think of loads that protect people. For other important loads, the operator decides based on a risk analysis.

The AREI calls those other loads critical installations. Its examples are a production line, a server room, a control room and a building for industrial livestock. The operator draws up the list, sets how long each load must keep working, and keeps the analysis for the inspection body.

Do you keep animals? Our article on keeping barn ventilation running in a power cut looks at alarms and generator starts on a farm.

When does a battery make the switchover time zero?

When your critical loads hang on a battery inverter/charger instead of directly on the transfer switch. A Victron Quattro takes over the supply in less than 20 milliseconds, fast enough for computers to keep running. The generator then starts in the background.

The Quattro has two AC inputs, for example the grid and a generator. It connects automatically to the source that is available. Its main output has the no-break function: when the grid disappears, the battery supplies the loads at once.

That changes the job of the generator. The generator no longer has to start at every cut. It only starts when the battery gets low. A Victron GX controller closes the start contact on conditions such as the battery state of charge or the load.

The generator also gets time to warm up. The GX gives the start signal, but the inverter/charger only connects the generator after the warm-up time. Before the engine stops, the loads move back to the battery for a cool-down. With the grid on one input and the generator on the other, the GX can keep the generator running until the grid returns.

Victron describes this set-up for grid-connected battery systems too: a diesel generator as back-up for long grid failures. Our media section shows a video of a Victron back-up system for an airport control tower. Our semi-off-grid system also uses an automatic transfer switch, between the grid and your own battery system. It works up to 75.6 kVA (3 x 400 V + N, 63 A).

Do you still need a UPS behind the battery? Our article on which inverter a business actually needs compares UPS, hybrid and battery inverters.

Can you keep your existing generator and add a battery?

Yes, and it usually gives the generator a smaller, better job. Several of our projects started with a generator that was already there.

In Maasmechelen the generator stood ready for emergencies. With the semi-off-grid system, the customer chooses the grid or the generator as the charger. Even on a home battery in Maaseik, a generator that was already there got an off-grid mode with a generator connection (9 kVA, 15.4 kWh).

On a farm in St-Georges-sur-Meuse, without a street connection, the generator used to run the whole site. After solar panels and a battery of 30 kVA and 46.2 kWh were added, it only serves as back-up.

A battery also works on days without a cut. It can shave your peaks and store solar power, while a fixed reserve stays locked for the cut. Our article on peak shaving with a battery explains the sizing. For a cold room, our article on how long a battery keeps a cold room cold works out the hours.

How often should you test the generator and the transfer switch?

At least once a year with a real switchover under load, and more often without load. For critical installations the AREI asks the operator for maintenance and supervision, with a switchover test under load at least once a year. The tests are documented.

The AREI names what supervision includes: the fuel level, the charge level of the batteries and the auxiliary equipment of the source. The American standard NFPA 110 goes further for emergency generators: exercise them at least once a month, for at least 30 minutes, with enough load.

A Victron GX can run an automatic periodic test run and track running hours and service intervals. With the grid switched off, a real test shows three things. Does the generator start? Does the switch move? Do all loads come back?

What happens when you ask us to look at your back-up?

We start with your loads and what a cut costs you, not with a device.

1. Step 1: Analysis and sizing. We list which loads cannot stop, which can wait a few seconds and which can stay off. Where data is missing, we measure your consumption for a short period first.
2. Step 2: Technical design. We decide what goes behind the battery, what stays on the generator and how the generator is started.
3. Step 3: Installation and commissioning.
4. Step 4: Monitoring and support. You follow the battery, the generator runs and the switchovers in the app and the VRM portal. Our monitoring software also shows consumption per phase.

Our business battery systems range from small SME installations to modular systems up to 1 MW. Back-up power and business continuity are one of their jobs.