Most switchgear ordering mistakes come from copying a part number off an old drawing instead of specifying from the feeder. The eight that recur are pole count, duty category, CT accuracy class, breaking capacity, coil voltage, temperature derating, missing accessories and the part number suffix. Each has a one-line rule that prevents it.
Where switchgear ordering errors come from
A wrong device rarely arrives because someone did not know the engineering. It arrives because the specification was copied: a part number from the last order, a line from an old panel drawing, a description from another project's bill of materials. The device was right for that board. Nobody checked whether it is right for this one.
The eight errors below are grouped by the field that goes wrong, not by who made the mistake. Each section explains what goes wrong, what it costs and the rule that prevents it.
The eight switchgear ordering errors at a glance
| Error | What goes wrong | Rule that prevents it |
|---|---|---|
| 1. Pole count | Three poles where four were needed, or the reverse | Count poles from the earthing system and the switching duty |
| 2. Duty category | A contactor rated for the wrong kind of load | Match the utilisation category to the load |
| 3. CT accuracy class | A metering CT on a relay, or a protection CT on a meter | Choose class from the job and burden from the circuit |
| 4. Breaking capacity | Icu below the prospective fault current | Icu at least the fault level at that point |
| 5. Coil voltage | A coil the control circuit cannot drive | State coil voltage in words, never as a code |
| 6. Derating | Nuisance tripping in a hot panel | Apply the maker's derating for the panel temperature |
| 7. Accessory omission | Shunt trips and contacts arriving late or never | Order every accessory on its device's purchase order |
| 8. Part number suffix | The right device in the wrong variant | Decode every character against the current catalogue |
Error 1: the wrong pole count
A three-pole MCCB and a four-pole MCCB of the same frame look alike on a bill of materials, and the difference is one character. The fourth pole switches the neutral. Whether you need it depends on the earthing system and on what the device does: a changeover between mains and a generator, for example, often needs the neutral switched so that the two sources do not share a neutral path, and some earth leakage schemes need it to work correctly. MCBs carry the same choice, between TP, TP+N and four-pole versions.
The cost is not only the device. A four-pole breaker is wider, so ordering the wrong one can mean a different busbar arrangement, a new cut-out or a door that no longer fits.
The rule: count poles from the system earthing and the switching duty of this feeder, not from the drawing you copied.
Error 2: the wrong duty category
Contactors are rated by utilisation category under IS/IEC 60947-4-1, and the same contactor carries different ratings for different duties. AC-1 covers resistive or slightly inductive loads such as heaters. AC-3 covers squirrel-cage motors started and stopped normally. AC-4 covers motors that are inched or plugged. Capacitor banks need a capacitor duty contactor, designed to limit the inrush current when a capacitor is switched in.
The errors run both ways. A contactor chosen on its AC-3 rating for a heater load is larger and costlier than it needs to be, because the same device's AC-1 rating is higher. An ordinary contactor switching a capacitor bank is the dangerous direction: the inrush can weld its contacts and shorten its life sharply.
The rule: name the load first — resistive, motor, inching or capacitor — and take the rating for that utilisation category from the maker's catalogue.
Error 3: the wrong CT accuracy class
A current transformer is built either to measure accurately at normal load or to stay accurate during a fault, and rarely both. Metering classes such as 0.5 and 1 are accurate around rated current but saturate at fault currents. Protection classes such as 5P10 and 5P20 keep their accuracy up to a multiple of rated current, so that a relay sees a fault as it is.
A metering CT feeding a protection relay may saturate exactly when the relay needs a true signal. A protection CT feeding a billing meter reads less accurately than the meter's class assumes. The opposite mistake is common too: specifying a tighter class than the job needs, which costs more and changes nothing. Burden matters as much as class, because the CT must drive the meter or relay plus the resistance of its leads within its rated VA.
The rule: choose the class from the job — meter or relay — and the burden from the device and the lead length it feeds, with the secondary current matching the meter or relay input.
Error 4: breaking capacity under-specified
Every breaker has a limit to the fault current it can safely interrupt: its ultimate breaking capacity, Icu, with a service breaking capacity, Ics, stated as a share of it. The prospective short-circuit current at the point of installation — set mainly by the supply transformer and the impedance up to that point — must not exceed what the breaker can break. Many MCCB frames are offered in more than one breaking capacity version, and the lower one is cheaper.
That is how the error happens: a lower version is ordered to save cost, or copied from a board fed by a smaller transformer. The breaker works normally for years and shows the mistake only when a fault exceeds what it can interrupt. Backup protection, or cascading, where an upstream device helps a downstream one, is valid only for the combinations a manufacturer has tested and published.
The rule: specify Icu at least equal to the prospective fault current at that point, at your system voltage, taken from a fault level calculation rather than the last board.
Error 5: the wrong coil voltage
A contactor coil must match its control circuit: 230 V AC, 110 V AC, 415 V AC and 24 V DC are all common in Indian panels, and a PLC-driven circuit is usually DC. The same contactor is offered with many coils, and the coil is selected by a suffix code. Those codes differ from maker to maker, so a code that means one voltage in one catalogue can mean another voltage, or nothing at all, in the next.
This is a transcription error, not an engineering one, and it costs a second freight leg and a stalled commissioning day. It applies equally to undervoltage releases, shunt trips and motor operators on breakers, which carry coils of their own.
The rule: write the coil voltage and type in words on every line — “230 V AC 50 Hz” or “24 V DC” — and check the suffix against that maker's current catalogue.
Error 6: ignoring temperature derating
Breakers are calibrated at a reference ambient temperature stated by the manufacturer. Inside a closed panel in an Indian summer, the air around the device can be far hotter than that reference, and a thermal release responds to the heat as if it were extra current. The result is nuisance tripping on a feeder that is not overloaded.
Mounting devices side by side, poor ventilation, undersized cables and high-altitude sites all add to the effect. Every maker publishes derating data for its range; the factors differ by product and are not interchangeable between brands.
The rule: estimate the panel's internal temperature, apply the manufacturer's derating for that range, and leave headroom between the derated rating and the load.
Error 7: accessories left off the order
A breaker or contactor is rarely installed alone. Shunt trips, undervoltage releases, auxiliary and alarm contacts, rotary handles, motor operators, terminal spreaders and phase barriers are separate part numbers, and they are often ordered at wiring stage, after the main devices have arrived. By then they may be on indent, and on some ranges certain internal accessories are fitted at the factory rather than on site.
This is one of the two errors in this list that are partly ours: a marketplace that lets accessories be ordered apart from their device makes it easy. The fix sits with both sides.
The rule: put every accessory on the same order as its device, checked against the schematic, and confirm whether each one can be fitted in the field.
Error 8: the part number suffix
A switchgear part number is a code. The base identifies the series and frame; the characters after it set poles, current, release, breaking capacity, coil voltage, terminals and sometimes the generation of the range. Two part numbers that differ by one character describe two different products, and a part number copied from an old order may belong to a superseded generation that is no longer stocked.
The rule: decode every character of the part number against the maker's current catalogue before ordering, and confirm that the series is still current. Our guide to switchgear brand cross-referencing explains why the same care applies when you move between makes.
What a wrong switchgear order really costs
The device is usually the smallest part of the bill. A wrong line costs in several places at once, and most of them never appear on an invoice:
Where the cost of a wrong switchgear order falls
| Cost | Where it shows up |
|---|---|
| Return or exchange | Freight both ways, and a restocking charge if the seller accepts the return at all |
| The second lead time | The right part on indent, starting from the day the error is found |
| Rework | Busbar, cut-out or wiring changes when the replacement differs in size or connection |
| Idle time | An electrician, a testing engineer or a whole commissioning team waiting for one line |
| Schedule | A handover date moved, or a panel shipped incomplete with the line to follow |
An opened or installed device is often not returnable, so the error caught at receipt costs far less than the one caught at commissioning. Check part numbers against the purchase order on the day the goods arrive, and run the authenticity checks in our guide to verifying genuine switchgear at the same time.
The pattern behind the eight errors
Read together, the eight errors have three causes, not eight:
- Copying instead of specifying. Pole count, breaking capacity and derating go wrong when a line is lifted from an earlier board whose fault level, earthing or ambient was different.
- Transcription. Coil voltage and suffix errors are typing and reading mistakes, made worse by codes that differ between makers.
- Splitting the order. Accessories ordered apart from their devices, and CTs ordered apart from the meters and relays they feed, arrive late or wrong.
The direction of each error is consistent too. Breaking capacity is under-specified, frame size and CT class are over-specified, and duty category is often not considered at all.
In our view that is good news: a short checklist run against the feeder catches all three causes, and none of it needs more engineering knowledge than the person ordering already has.
A pre-order checklist for switchgear
Run these eight checks against the feeder, not against the old drawing, before any switchgear order is sent.
- Poles counted from the earthing system and the switching duty.
- Contactor ratings chosen for the load's utilisation category.
- CT class from the job, burden from the circuit, secondary matched to the input.
- Icu at least the prospective fault current at the point of installation.
- Coil voltages written in words on every line, with suffix codes checked.
- Derating applied for the panel's internal temperature.
- Every accessory on the same order as its device.
- Every part number decoded against the current catalogue.
A list that passes all eight is ready for an RFQ. Our post on the 20 most used electrical components covers the ratings to specify for each item, our guide to reading a switchgear quotation covers what to check when quotations come back, and our guide to the electrical procurement process explains what happens next.
This checklist takes ten minutes, and in our view it saves the most expensive conversation in procurement: the one about a device that is already on site and wrong.
Our position
Eight errors, but not eight causes. Nearly all of them come from the same act: a part number lifted off an old drawing or a previous order and sent to us as a specification. The device was correct for that board. Nobody asked whether it is correct for this one. Ignorance is the smaller problem.
The direction is consistent. Breaking capacity goes down, frame size and accuracy class go up, and duty category is simply not considered — AC3 contactors on resistive loads, ordinary duty where capacitor duty was needed. Coil voltage and suffix errors are not engineering mistakes at all. They are transcription, and each one costs a second freight leg.
Two of these eight are ours. We quoted what the buyer typed without asking what the feeder does, and we let accessories be ordered separately from the device, so shunt trips and auxiliary contacts arrive after the panel is wired or not at all. Our own record of these errors also flatters us: it holds only the errors that came back to us. A wrong device that went in and passed inspection is not in it.
Questions buyers ask
What should I do if the wrong MCCB is delivered?
Do not install it or unpack it further. Check the part number against your purchase order and the seller's quotation, then tell the seller in writing, with photographs of the label. Whether it can be returned, and at whose cost, depends on the order terms and on whose specification was wrong.
Can an AC-3 rated contactor be used for an AC-1 load?
Yes. A contactor's AC-1 rating is higher than its AC-3 rating, so a contactor chosen for a resistive load can often be smaller than one chosen on AC-3. Use the AC-1 figure from the maker's catalogue.
Can a metering CT be used for protection?
It is not recommended. Metering CTs saturate at fault currents, so a relay fed from one may not see the fault correctly. Use a protection class CT, such as 5P10 or 5P20, for relays.
What does the suffix on a switchgear part number mean?
The characters after the base part number select the variant: poles, rated current, release, breaking capacity, coil voltage, terminals or generation. Each maker uses its own scheme, so decode a suffix only from that maker's current catalogue.
How do I choose between a three-pole and a four-pole MCCB?
Choose four poles where the neutral must be switched — for example on a changeover between sources with separate neutrals, or where the earthing system or an earth leakage scheme requires it. Otherwise three poles with a solid neutral link is usual. The single-line diagram should show which.
How do I find the fault level at my panel?
From a short-circuit calculation based on the supply transformer's rating and impedance and the impedance of the cables and busbars up to the panel. Your electrical consultant or panel builder can provide it, and it belongs on the single-line diagram.
