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Search for this topic and you’ll get the same sentence a dozen times: PC class only switches, CB class switches and protects. That’s true. It’s also where almost every article stops, which is unfortunate, because it’s the part of the decision that takes ten seconds. The part that actually bites people comes later.
This guide covers the ten-second answer quickly, then spends the rest of its time on the three things that decide whether your choice works in the field: what each type does when a fault actually happens, what you still have to specify after you pick PC class, and why the whole calculation shifts when your backup source is a solar inverter or a battery rather than the grid.
If you’re still working out what size of switch you need in the first place, start with our ATS sizing guide and come back here. If transfer switches are new to you entirely, what a transfer switch is and how it works covers the basics.
PC class vs CB class ATS: Quick Answer
A PC class ATS is a switching device. It moves the load between two sources and can hold on through a short circuit without coming apart, but it can’t clear the fault itself — something upstream has to do that. A CB class ATS is built from circuit breakers, so it can both switch the load and interrupt a fault on its own.
Pick PC class when you want the switching function to be fast, compact, and mechanically independent of your protection scheme. Pick CB class when you want switching and protection in one box and you’ve accounted for what happens if that box trips. Both are defined under IEC 60947-6-1, so neither is a downgrade — they’re answers to different questions.
The Short Version, With an Analogy That Actually Holds Up
Think of a railway junction. A PC class ATS is the set of points that redirects the train onto a different track. Its job is to move the train, and to be strong enough that a derailment somewhere down the line doesn’t rip the points apart. Stopping the runaway train is somebody else’s job, further up the track.
A CB class ATS is a set of points with an emergency brake built into the mechanism. It can redirect the train and stop it. Convenient, and one less thing to install. But now think about what happens when that brake fires: the same hardware that was supposed to move the train onto the other track has just committed itself to stopping everything.
What Actually Happens During a Fault
PC class, downstream short circuit. The fault current flows through the ATS. The switch is rated to make and withstand that current for a defined time without its contacts blowing apart or welding shut. Meanwhile the upstream protective device — a breaker or fuse set — clears the fault. Once it’s cleared, the ATS is still a functioning switch. It can still transfer.
CB class, same short circuit. The breaker inside the ATS senses the fault and trips. The fault is cleared faster and more locally, which is genuinely the advantage of CB class. But the breaker that just tripped is the switching element. Depending on the design, the control logic, and how the trip is reset, you may now have an ATS that has protected your installation and simultaneously lost the ability to transfer until somebody resets it.
Worth thinking through: if the load on the other side of your ATS includes anything that needs to stay powered during a fault event elsewhere in the system, ask your supplier directly how their CB class unit behaves after a trip, and whether transfer capability is restored automatically or requires manual reset. This isn’t a spec-sheet number. It’s a behaviour question, and you have to ask it.
For most commercial and industrial installations this is perfectly manageable — you plan for it, and the local fault protection is worth having. For continuity-critical loads, it’s the reason a lot of designers go PC class and handle protection separately.
If You Pick PC Class, You Are Not Finished
PC class switches are tested to a conditional short-circuit rating. Conditional means exactly what it sounds like: the rating holds on the condition that a specific type and rating of upstream protective device is present. The manufacturer tests the ATS with a particular short-circuit protective device and certifies the pair. Swap in a different breaker with a different let-through characteristic, and you no longer have the rating you think you have.
So when you specify a PC class ATS, ask your supplier for three things, in writing:
- The conditional short-circuit current rating, and the exact upstream protective device it was tested with
- The maximum time the switch can carry fault current without damage
- Whether the rating changes across the current range within the same product series
Warning: “It’s rated to 25kA” is not an answer to this question. It’s the first half of an answer. The second half is “with what upstream device,” and if a supplier can’t give you that, the rating isn’t verifiable.
This is dull, and it is also the single most common way a correctly-chosen PC class ATS ends up incorrectly installed.
Why Solar, Storage and EV Charging Change the Calculation
The PC-versus-CB debate was built around a world where both sources are effectively unlimited: the utility grid on one side, a rotating generator on the other. Both can dump enormous fault current into a short circuit. That’s the whole reason the protection question is so central.
Now replace the backup source with a solar inverter or a battery inverter, and something fundamental shifts. Inverters are current-limited by their own electronics. When a fault appears, an inverter doesn’t heroically pour current into it the way a grid transformer or an alternator does — it hits its limit almost immediately and, in many cases, shuts itself down.
This has two practical consequences:
The fault current available depends on which source is connected. The same downstream short circuit looks completely different to your protection scheme depending on whether the ATS is currently sitting on grid or on inverter. Protection that coordinates beautifully against grid fault levels may not see enough current from the inverter side to trip at all, or may trip far more slowly than the coordination study assumed.
The “CB class gives you protection” argument gets weaker on the inverter side. A thermal-magnetic trip unit needs current to work with. If the inverter limits fault current to a level not far above its own rated output, that trip unit may be operating in a region where it’s slow or ineffective. The integrated protection is still real on the grid side; it may be much less useful on the backup side.
None of this makes CB class wrong for solar and storage. It does mean the decision can’t be copied from a generator-backup design, and it means the inverter’s actual fault-current contribution needs to come from the inverter manufacturer’s datasheet, not from an assumption.
Verify this one for your specific project: inverter fault-current behaviour varies enormously between manufacturers, models, and firmware versions, and grid-code requirements in some regions mandate specific fault-ride-through behaviour that changes it further. Get the number from the inverter datasheet. Don’t let anyone, including this article, hand you a rule of thumb for it.
Our guide to solar transfer switch applications covers the broader PV context, and if your system also has rapid shutdown requirements, rapid shutdown solar design explains how the ATS fits into the isolation sequence.
Side-by-Side Comparison
| PC Class | CB Class | |
|---|---|---|
| Primary job | Transfer the load between sources | Transfer the load, and protect the circuit |
| Can it break fault current? | No. Withstands it; upstream device clears it | Yes, with its own trip unit |
| Built from | Integrated switching mechanism with interlock and arc chutes | Two circuit breakers plus an interlocking transfer mechanism |
| Typical transfer speed | Generally faster | Generally slower, because a breaker mechanism has more to do |
| Physical size | More compact for a given current | Larger, taller |
| After a downstream fault | Still able to transfer | May require reset before it can transfer again |
| Extra equipment needed | Yes — a coordinated upstream protective device | Often none at the transfer point |
| Short-circuit rating type | Conditional (tied to a specified upstream device) | Rated independently (Icu / Ics / Icm) |
| Suits | Continuity-critical loads, tight enclosures, systems with an existing protection scheme | General industrial and commercial projects, installations where one integrated device simplifies the build |
Two things this table won’t tell you: which one is cheaper (it depends far more on current rating and market than on class), and which one is “better” (neither, genuinely).
A Decision Checklist
Work through these in order. The first one that gives you a clear answer usually settles it.
- Is there already coordinated overcurrent protection at or near the transfer point? If yes, PC class avoids paying twice for the same function.
- Can your load tolerate the ATS being out of action after a downstream fault? If no, that’s a strong argument for PC class.
- Is enclosure space tight? PC class is generally more compact for the same current.
- Does the project specification or local code mandate protection at the transfer point? If yes, CB class, and the decision is made for you.
- Is the backup source an inverter rather than a generator? If yes, get the inverter’s fault-current contribution before assuming integrated protection will behave as expected.
- Is this a life-safety circuit (fire pump, fire lift, emergency lighting)? If yes, neither class alone answers the question — you need a configuration certified for fire-pump service under the relevant annex of IEC 60947-6-1 or UL 1008, and the local fire code governs.
Mistakes We See Regularly
Treating “CB class” as the premium option. It isn’t a tier. It’s a different architecture with a different failure mode. Plenty of demanding installations specify PC class deliberately.
Accepting a short-circuit rating without the conditions attached. Covered above, and worth repeating because it’s the most common one.
Assuming the class determines the transfer time. It influences it, but the controller logic, the sensing thresholds, and the deliberate transfer delays usually matter more. If transfer time is critical, ask for the actual figure for the actual model.
Copying a generator-backup design onto a solar or battery system. The protection logic doesn’t transfer cleanly, for the reasons in the inverter section above.
Forgetting that class says nothing about pole count, voltage, or transition type. These are independent choices. A PC class switch can be 2P or 4P, open or closed transition. Don’t let the class decision quietly stand in for the others.
MOREDAY’s Range
MOREDAY’s ATS products span both approaches across the full ATS range: the MDQ2-63 and MDQ3-63 at the 63A tier, the MDQ5R-100 for mid-size solar and storage systems, and the MDQ8 isolation type for larger commercial and industrial installations. For sites where manual changeover is acceptable, the NSIS AC Changeover Switch covers the same ground without the automation.
MDQ2 MDQ3 MDQ5 MDQ8 Rated Current 6A – 63A 6A – 63A 16A – 100A 100A – 3200A Rated Voltage 230V / 400V 110V / 230V / 400V 230V / 400V 230V / 400V Poles 2P / 4P 2P / 4P 2P / 4P 2P / 4P Switching Time ≤3s ≤3s ≤3s ≤3s Control Mode Automatic / Manual Automatic / Manual Automatic / Manual Automatic / Manual Frequency 50/60Hz 50/60Hz 50/60Hz 50/60Hz Mechanical Life ≥8000 times ≥8000 times ≥8000 times ≥8000 times
If you’d like help matching a class and configuration to a specific project, talk to MOREDAY’s engineering team with your single-line diagram and source details.
FAQ
Is CB class better than PC class?
No. They solve different problems. CB class puts protection and switching in one device; PC class keeps them separate so a fault event doesn’t take out your switching capability. Which is better depends entirely on your system.
Does PC class mean the switch has no protection at all?
It means the switch doesn’t interrupt fault current itself. It’s still rated to carry and withstand it. Protection is provided by a coordinated device upstream, and that device has to be the one the ATS was tested with.
Which class is faster?
PC class is generally faster, because there’s less mechanism to move. But controller settings and deliberate transfer delays usually have a bigger effect on real-world transfer time than the class does.
Can I put a PC class ATS behind any circuit breaker?
No, and this is the most important answer on this page. A PC class ATS’s short-circuit rating is conditional on a specified upstream device. Substituting a different one invalidates the rating.
What about CC class? I’ve seen it mentioned.
Some national standards define a third category. In practice PC and CB cover the overwhelming majority of commercial products, and if you’re working to a national standard that defines additional classes, that standard is the authority, not a general article.
Does the class affect whether I need a 4-pole switch?
No. Pole count is driven by your grounding and neutral arrangement, not by the class. The two decisions are independent.
The Bottom Line
The one-line difference between PC and CB class is easy. The consequences are where the engineering lives: a CB class switch that trips has spent its switching ability to protect you, and a PC class switch is only as good as the upstream device it was certified with. Get those two things straight and the choice is usually obvious.
And if the backup source is an inverter rather than a generator, go and get the fault-current figure from the datasheet before you decide anything. That single number changes more about this decision than the class label does.
Standards referenced: IEC 60947-6-1, Low-voltage switchgear and controlgear — Transfer switching equipment; UL 1008, Standard for Transfer Switch Equipment.
