Short answer: a PC class transfer switch changes the load with one purpose-built contact mechanism and brings no overcurrent protection of its own. A CB class transfer switch is two circuit breakers — usually moulded-case — coupled to a controller, so the transfer path also protects the cable and gives a visible break for maintenance. If the fault has to be cleared at the transfer point, choose CB class. If the feeder is already protected upstream and you want the fastest, most compact changeover, PC class does the same job with less hardware.
We are an independent trading company — not a manufacturer, and not an appointed distributor of any brand — so every rating on this page is quoted per exact ordering code, and the numbers you sign off should come from the datasheet of the code you are actually buying.
One standard, two constructions
Both classes sit under IEC/EN 60947-6-1, the transfer switching equipment standard. The standard does not rank them; it defines test duties. The utilisation category tells you what the device was tested to do: the AC-33 family covers mixed loads including motors, and the letter that follows tells you whether the device is rated for frequent transfer (A), infrequent transfer (B), or carries an intentional delay (i). Two devices can both be “AC-33” and still differ in how many operations they survive, which is exactly why the suffix exists.
The North American route is different. UL 1008 is the listing standard for transfer switch equipment there, and a device certified to IEC 60947-6-1 is not automatically acceptable under UL 1008. If the destination is the US, Canada or a project specified to UL, that listing question has to be settled before anything else on this page matters.
PC class against CB class, side by side
Read the first row first. Everything else follows from what is actually moving the current.
| Property | PC class (single body) | CB class (breaker based) |
|---|---|---|
| What transfers the load | One integral contact assembly driven by a motor operator or solenoid, with a mechanical latch for each position | Two circuit breakers, each closing onto its own source, driven by motors or by the breaker’s own mechanism |
| Overcurrent protection | None. The device switches; it does not protect | Yes — the two breakers are the protection, so the cable downstream is protected at the transfer point |
| Fault current behaviour | Rated for a withstand and a making capacity, not for clearing a fault | Each breaker carries a breaking capacity (Icu) at its rated voltage, and clears faults on its own source |
| Isolation for service | Contacts may not give a visible break. Add an upstream isolator | Breakers can be locked off, and most frames give a positive visible indication |
| Typical frame range | Module sizes at the low end up to large frames — the CHINT NXZM / NXZBN / NXZHM PC-class family runs to the 1600 A level | Commonly built on small frames — the CHINT NXZB-63 / NXZHB-63 is an In ≤ 63 A frame, 2P / 4P, AC 50 Hz, motor operated with manual operation and reset |
| Footprint and cost at low ratings | One body, fewer terminations, usually the cheaper answer below a few hundred amps | Two breakers plus a controller plus interlocking, so cost and panel space rise quickly |
| Where it is the natural choice | Generator back-up on a feeder already protected by an MCCB or MCB; compact panels; retrofit where space is fixed | Main incomer transfer; installations where the authority wants the transfer device to be the protective device; maintenance isolation as a designed function |
The interlock is the safety case, not a feature
The only thing standing between a generator and the public supply is the guarantee that both sources are never closed at the same time. In a PC class device that guarantee lives inside the mechanism: a mechanical interlock plus, on most frames, a position switch that reports the state to the controller. In a CB class device it is split between the two breakers and the controller, which means the interlock has to be wired, tested and maintained as its own subsystem.
A failed interlock is not a nuisance trip. It parallels two sources through the transfer device, backfeeds the transformer or the street, and puts line workers at risk. When a changeover switch “sometimes shows both sources live”, stop using the panel and inspect the interlock mechanically before touching the controller settings.
Choosing by site condition
| Situation | Class | Reason |
|---|---|---|
| Shop, office or small building with a generator; incomer already has an MCCB | PC class | Protection already exists; you are only buying the transfer function |
| Main incomer where the transfer device is expected to clear faults | CB class | The breakers are the protection; one device does two jobs |
| Above roughly a few hundred amps | PC class normally | Two large breakers plus controls is a heavy, expensive assembly; a large PC frame is simpler |
| Service staff need a lockable, visible break | CB class, or PC class plus a lockable isolator | Isolation is a designed function, not a by-product |
| Motor-heavy load bus | Either, but check the AC-33 duty and the motor’s ride-through | The transfer break may drop every contactor on the bus |
| North America, or a UL-specified project | Whichever carries the UL 1008 listing | IEC certification does not transfer |
Three-pole, four-pole and the neutral
The poles question is decided by the earthing system, not by habit. A switched neutral (4P) is needed where the neutral must be broken to keep the neutral and protective earth bonded at one point only — typically a generator with its own neutral-to-earth link, or a TN-C-S supply where paralleling neutrals would create a second path. A solid neutral (3P plus a solid N) is cheaper and common where the neutral is not switched by the source. Getting this wrong shows up later as nuisance RCD tripping or as circulating current in the neutral, and it cannot be fixed in software.
Frequency is the other quiet trap. DC coils aside, controls are rated for a supply frequency: the NXZB-63 frame is documented at AC 50 Hz. On a 60 Hz site, confirm the coil and controller rating for the exact code before ordering rather than assuming the label covers both.
Mistakes that come back as warranty claims
Using a PC class device as a breaker. It has no overcurrent element. The upstream breaker must be sized for the cable, and the PC device must be rated to withstand the fault current that the upstream device lets through.
Specifying CB class and leaving the controller at factory defaults. Transfer delay, return delay, source-failure thresholds and generator start signals all need to be commissioned against the actual site.
Assuming any changeover will ride through. Every break-before-make transfer opens the bus for a measurable time. Check the hold-in time of each contactor and the ride-through of each drive instead of assuming the load will not notice.
Forgetting the neutral. Four-pole or solid neutral is a system decision made at design, not at commissioning.
What to send us for a quotation
Number of poles and neutral arrangement · rated operational current and the load mix (motors, drives, lighting, IT) · prospective fault current at the transfer point · the two sources (grid plus generator, or two transformers) · required transfer and return delays · control voltage · whether a visible lockable break is required · destination country and the certificate the project accepts · quantity. You get unit price, packing data and the certificate document for the exact ordering code. Request a quotation.
FAQ
Q: Is a PC class changeover always cheaper than CB class?
A: At the same current and pole count it usually is, because you are buying one mechanism instead of two breakers plus a controller. The comparison only holds if the feeder is already protected upstream — if you have to add a breaker to make the PC device safe, the saving narrows or disappears.
Q: Can I build a CB class unit myself from two interlocked breakers and a controller?
A: Mechanically you can assemble something that works, but it is not a tested transfer switch. IEC 60947-6-1 tests the combination as a unit for transfer duty, and a site-built assembly has no such evidence. Where the authority or the insurer asks for the certificate, the assembly fails the question.
Q: Which class for a small generator back-up panel?
A: PC class in most cases. The incomer breaker already protects the cable, the load is modest, and a compact PC device keeps the panel small. Move to CB class when the transfer point itself has to clear faults or when lockable isolation is a requirement.
Q: Does the changeover switch replace an isolator?
A: Not automatically. A PC class device may not give a visible break, so a lockable isolator upstream is still the maintenance answer — the NH4 modular switch disconnector is the usual match at the modular end. A CB class device does give a lockable break through its breakers.
Q: Can the same unit be used at 50 Hz and 60 Hz?
A: Only if the ordering code says so. Coils and controllers are frequency-specific on many frames, and the 63 A CB-class frame is documented at AC 50 Hz. Tell us the site frequency and we quote the code that matches it.
Related: ATS, SPD and isolators · ATS vs manual changeover · MCB vs MCCB vs ACB · Selectivity between MCB and MCCB · Voltage and frequency by country · Contactor coil 50 Hz vs 60 Hz · How to verify genuine product