After the correct interrupting rating is verified, the Westinghouse BK oil circuit breaker can be compared with available fault current without mistaking its 600 A continuous rating for fault-clearing duty. The reported nameplate values—7,500 V, 600 A, type BK, and manufacture circa 1940—do not by themselves define interrupting capacity.
Current, thermal load, and interruption duty
The 600 A value describes continuous current unless another nameplate label explicitly assigns it a different function. It represents the normal-current thermal duty of the conducting path. Interrupting duty concerns the much larger fault current that the contacts, oil, arc-control structure, bushings, and operating mechanism must clear without loss of insulation or mechanical integrity.
The number that matters is the prospective short-circuit current at the breaker location, evaluated at the applicable system voltage and against the rating basis printed in the breaker documentation. During interruption, current produces contact heating and electromagnetic force while the opening contacts establish an arc in oil. After current interruption, the contact gap must withstand the recovering system voltage. This is heat, force, arc control, and dielectric recovery—not the normal 600 A load limit.
| Quantity | Meaning | Where to read it |
|---|---|---|
7,500 V |
Reported breaker voltage rating; not an interrupting-capacity value | Main nameplate |
600 A |
Reported continuous-current rating unless the label states otherwise | Main nameplate |
| Interrupting current or capacity | Fault-clearing limit at the stated rating basis | Nameplate, matched drawing, instruction book, or manufacturer archive |
| Available fault current | Duty imposed by the installed power system | Current short-circuit study |
| Style, serial, or model suffix | May identify a construction or rating variant | Nameplate and mechanism plates |
Rating-identification approaches
Three approaches can produce useful information, but they do not carry equal authority. Use the direct nameplate or a document tied to the exact breaker before relying on nomenclature or corporate lineage.
| Approach | Strength | Limitation | Decision |
|---|---|---|---|
| Read the complete nameplate | Identifies the installed unit and may state interrupting amperes or MVA directly | Plate may be dirty, damaged, painted, or use historical terminology | First choice |
| Decode type, model, or style | Some historical breakers embed an interruption class in the designation | Numbering rules are manufacturer- and product-specific | Use only with a matching Westinghouse document |
| Request an archival identification | Can connect serial/style data to drawings or rating tables | The applicable successor organization is uncertain for this product | Use when local records are incomplete |
A historical example from another manufacturer is GE FK-255-500, where 500 denotes a 500 MVA interruption value. That example proves only that encoded ratings existed; it does not establish a decoding rule for a Westinghouse BK designation.
Nameplate-first recommendation
Treat the interrupting capacity as unidentified until a direct marking or matched document supplies it. A breaker type, rated voltage, and continuous-current value cannot be combined to calculate an unknown interrupting rating. Voltage defines one boundary condition, while interrupting capacity depends on the breaker design and its assigned rating.
Start with the entire main nameplate rather than the large characters alone. Capture every prefix, suffix, punctuation mark, serial number, style number, frequency marking, pole information, and interrupting-duty field. Photograph the mechanism and control plates as well, because replacement mechanisms or controls can carry identifiers that differ from the tank-mounted nameplate.
If local drawings and manuals do not resolve the rating, send the complete identification package to official technical-support channels. ABB in the United States has been identified as a possible archival path for the former Westinghouse high-voltage breaker line. Eaton/Cutler-Hammer is another possible path because it manufactures some switchgear lines of Westinghouse design. Contacting either company is an identification inquiry, not proof that it owns documentation for this particular Canadian Westinghouse breaker.
Identification procedure
- Have qualified personnel isolate the breaker under the facility electrical-safety procedure. Account for stored mechanical energy, control power, and the hazards associated with the oil-filled apparatus before approaching or cleaning nameplates.
- Photograph the breaker from several angles, including the complete nameplate, tank, bushings, operating mechanism, control cabinet, and every secondary plate. Use oblique lighting to reveal shallow stamped characters.
- Transcribe the visible data exactly. Preserve the type
BKdesignation and all adjacent letters or numbers; a suffix can separate rating variants that otherwise look identical. - Search site records for drawings, purchase documents, test reports, spare-parts lists, and instruction books carrying the same serial, style, or complete type designation.
- Check whether the nameplate expresses interruption in amperes or MVA. Record the associated voltage and any wording that defines the duty; a capacity value without its rating basis is incomplete.
- Submit the photographs and transcription to official support channels for ABB and Eaton/Cutler-Hammer if the plant records contain no matched rating. Ask for documentation tied to the exact serial or style rather than a general rating for the
BKfamily. - Retain the returned drawing, table, or written identification with the equipment record. Reject a catalog value when its type suffix or construction does not match the installed breaker.
Capacity conversion and application check
Historical records may express interrupting capacity as apparent power rather than current. Convert only after identifying the system topology and confirming that the documented MVA value uses the same voltage and rating basis.
If the documented value applies to three-phase line current, use:
kVA = sqrt(3) × V_LL × I_line / 1000
I_line = kVA × 1000 / (sqrt(3) × V_LL)If it applies to a single-phase circuit, use:
kVA = V × I / 1000
I = kVA × 1000 / VThe reported information does not state the phase topology or provide an interrupting MVA value, so no interrupting-current result can be calculated from it. Once the authentic rating is found, compare it with a current short-circuit study at the breaker terminals. Match units and rating basis before comparing values, and examine interrupting duty separately from closing, momentary, or mechanical duties listed in the recovered documentation.
Verification and recurring pitfalls
Verification has two independent parts. First, confirm that the documented interrupting rating exceeds the applicable calculated duty under the same basis. Second, determine whether this circa-1940 breaker remains physically capable of its assigned duty. A nameplate rating does not certify present condition.
Have a qualified breaker service organization evaluate the oil, insulation, contacts, bushings, operating mechanism, timing, and maintenance history using procedures applicable to the identified construction. Record test results against the matched service documentation. A successful no-load opening operation does not demonstrate fault-interruption capability.
| Pitfall | Why it fails | Correct action |
|---|---|---|
Using 600 A as interrupting current |
It confuses continuous heating duty with fault-clearing duty | Locate the separately identified interrupting rating |
Calculating capacity from 7,500 V alone |
Voltage supplies no fault-current limit | Recover a rated current or MVA value |
Applying the GE FK-255-500 naming rule |
Another product family can use a different code structure | Require a Westinghouse document matched to the full designation |
| Accepting a family-level catalog value | Type suffixes may represent different interrupters or duties | Match serial, style, suffix, and construction |
| Equating operational testing with interrupting capability | Mechanical motion does not validate arc-control or dielectric performance | Complete the rating check and condition assessment separately |
Frequently asked questions
What happens if 600 A is used as the interrupting rating?
The application check becomes invalid because 600 A is the reported continuous-current rating. Obtain the separately marked interrupting amperes or MVA value before comparing the breaker with available fault current.
What happens if the Westinghouse BK nameplate lists MVA instead of amperes?
Convert MVA to current only after confirming the documented voltage, phase topology, and rating basis. For three-phase line current, use I_line = kVA × 1000 / (sqrt(3) × V_LL); use I = kVA × 1000 / V for a documented single-phase case.
What happens if the BK style number appears to contain the rating?
Use that interpretation only when a matching Westinghouse drawing, manual, or rating table defines the code. The GE FK-255-500 example cannot be transferred to the BK numbering system.
What happens if the nameplate and plant records cannot verify the rating?
Stop the application decision and keep the breaker from duties that depend on an assumed interrupting capacity. Escalate with full photographs, serial/style data, and installation records to official ABB and Eaton/Cutler-Hammer support channels, then use only documentation matched to the installed unit.