A buyer searching for MCB is usually trying to stop a bad substitute. Low-voltage distribution gear is specified by the fault level, the current, the poles, and the enclosure it sits in. The notes below are selection criteria for an MCB. They are not a catalog promise. Confirm every rating on the current datasheet, drawing, or test report for the revision you will receive.
What has to be true of MCB
Low-voltage distribution gear is specified by the fault level, the current, the poles, and the enclosure it sits in. For an MCB, start from the duty, then the interface to the rest of the machine or the room, then the document that proves the claim. A lower price that leaves one of those blank is a different product.
MCB is easy to confuse with Distribution Transformer. If the quote could be read as either, add the line that separates them: the rating, the fitment, or the standard named below. Link the drawing number to that line so receiving inspection can reject the wrong one.
Numbers that change the quote
| Line | What to write for MCB |
|---|---|
| Mounting | DIN rail, panel, or a stated enclosure with its IP code. |
| Coordination | What sits upstream and downstream so the combination is selective. |
| Rated current | Continuous current at the ambient inside the panel. |
| Breaking capacity | The fault current the device must interrupt, in kiloamperes. |
| Poles and trip | Poles, curve or trip unit, and residual current if people are protected. |
| Voltage and frequency | The real mains, single or three phase. |
How to compare two offers
Lay both quotes for MCB on one sheet and line up mounting, coordination, and rated current. If a cell is empty, the offer is not finished. Ask for the missing line in writing. When those match, the honest difference is lead time, the test evidence, and who carries the warranty. Do not average a fully specified MCB with a cheaper line that omitted mounting.
Documents worth having before the deposit
IEC 60947 is the usual family for low-voltage switchgear. Ask which part of it the declaration uses. The file should name MCB and the revision, not only the supplier’s homepage. A factory quality certificate such as ISO 9001 can sit beside that file. It does not replace it.
Failures that show up after delivery
- Sizing a breaker from the load current and ignoring the fault level.
- Mixing a person-protection device with a plain miniature breaker.
- Cutting an enclosure and assuming the IP code survived.
On an MCB, the expensive version of the first mistake is discovering it after the goods are in the crate. Put the rejection condition in the purchase order while the supplier can still restate the offer.
A worked RFQ line
Use this shape, then replace the brackets with your duty: “MCB — Write current, breaking capacity, poles, voltage, and the enclosure code.” That sentence is a template, not a rating of any brand. If a supplier’s datasheet uses different units, convert them onto this sheet so the next offer can be compared.
Where words like OEM / custom belong
Searches often add a constraint to MCB: OEM / custom, or stainless / corrosion. Keep those on this page. OEM or custom means a drawing, a revision, and a statement of who owns the tool. Without those, you are buying the catalog part. Name the alloy or the coating and the chemical it must survive. Stainless is a family, not a guarantee. If you need a third constraint, add it as another line on the same drawing. A second URL that only swaps the adjective does not give a buyer a second fact.
Related in this section: how to specify Distribution Transformer.
Questions buyers actually ask
What should an RFQ for MCB include?
Write current, breaking capacity, poles, voltage, and the enclosure code.
Is a cheaper MCB the same part?
Only if mounting, coordination, and the documents match. A missing line is a different offer.
Do waterproof, OEM, or IP words need their own page?
No. For MCB they are extra lines on this specification. Ask for the test or the drawing that gives the word a value.