7 ABB Contactor Mistakes That Cost Me Time & Money (Learn From My Errors)

Getting Started: Why I'm Writing This

I'm an industrial controls technician handling customer orders and installations for about 6 years now. I've personally made (and documented) 14 significant mistakes involving ABB contactors and motor controls, totaling roughly $12,400 in wasted budget and rework. Now I maintain our team's internal checklist to prevent others from repeating my errors. This article covers the most common and costly ABB contactor mistakes I've seen — and made.

1. What's the biggest mistake people make when ordering an ABB contactor?

Assuming the coil voltage matches your control system. This is mistake #1. I see it constantly. Someone specs an A75-30-11-84 without checking if they need 24 VDC, 110 VAC, or 230 VAC. That's not just a supply problem — it's a system integration failure.

In my first year (2017), I made this exact error on a rush order for a packaging line. I ordered six A50-30-10 contactors with 24 VAC coils. The customer's system was 24 VDC. The contactors clicked but wouldn't hold in. That mistake cost $890 in redo plus a 1-week delay. The lesson: always confirm the coil voltage spec in writing before placing the order. Don't assume.

2. How do I know if an ABB contactor will fit in my panel?

You can't just guess based on the frame size. I assumed 'same frame size' meant identical mounting dimensions across all variants. Didn't verify. Turned out auxiliary contact blocks, especially side-mounted ones, change the profile significantly.

On a 20-contactor retrofit job in September 2022, I ordered AF09-30-10-11 contactors to replace older units. The footprint was right, but the added top-mounted auxiliary block made the total height 8mm too tall for the panel door clearance. We had to rework the panel layout (3 days of lost production time, $2,100 in labor).

Check the ABB contactor manual before ordering. Always download the datasheet. Pay attention to the "overall dimensions" diagram, not just the mounting hole centers. This is especially critical for reversing contactors or any setup with mechanical interlocks.

3. What's the difference between a relay and an ABB contactor? (And why does it matter?)

Honestly, I've never fully understood why some engineers use these terms interchangeably, because they aren't. A relay is a low-power switching device. A contactor is designed for high-current, high-inductive loads — like motors, lighting banks, and heating elements.

The practical difference for you: don't substitute a relay for an ABB definite purpose contactor on a motor start circuit. I tried this once on a 2 HP blower motor. The relay contacts welded closed within 30 cycles. Cost me $180 in parts and a fire risk.

When you're dealing with high inrush currents (think motors, transformers), use a contactor properly rated for the application. An ABB A-line or AF-line contactor is built for this. A general-purpose relay isn't.

4. Why does my ABB contactor hum or buzz loudly? Is it broken?

Not necessarily broken, but definitely something to investigate. In most cases, it's not the contactor itself — it's the control voltage. A humming contactor often indicates a low voltage condition or a bad coil.

On a Lister Petter diesel generator project in Q1 2024, I had a customer complaining about a buzzing AF30-30-10 contactor at the 3-minute mark after startup. The generator voltage was sagging to about 85% of nominal during load pickup. The contactor's coil couldn't maintain full magnetic hold, causing the humming.

The fix wasn't replacing the contactor — it was adding a voltage protection relay that delayed the contactor closure until the generator voltage stabilized. This is a classic interaction I see all the time.

5. What's the right way to test an ABB contactor?

A lot of people just jumper power and hope. That's a good way to let out the magic smoke. You need a methodical approach. I learned this the hard way after frying a coil.

Here's our 5-minute test sequence from my checklist:

1. Continuity check (coil off): Main contacts should be open (OL on your meter).
2. Coil resistance check: Measure across A1-A2. You're looking for a reasonable resistance (typically 100-500 ohms for small contactors, lower for larger ones). A short (0 ohms) or open (OL) means a bad coil.
3. Power the coil (A1-A2): Apply control voltage. You should hear a solid click.
4. Continuity check (coil energized): Main contacts should now show low resistance (less than a few ohms).
5. Check auxiliary contacts (13-14, 21-22, etc.): Make sure normally-open contacts close and normally-closed contacts open under power.

This bit me on a project with a vacuum contactor. I only checked the main contacts, didn't bother verifying the auxiliary contacts. The interlock signal never arrived. The ABB contactor manual for that model had the pinout clearly marked, but I was in a hurry. 30 minutes of head-scratching later...

6. Should I worry about the difference between a relay and a contactor when choosing a voltage protection relay?

Absolutely, yes. A voltage protection relay (VPR) is designed to work with starter coils, not line current. The VPR itself uses a relay output, but the load it controls is the contactor coil.

I made the assumption error on a job in Lincoln, UK, on a temporary generator setup. I sourced a VPR that had a relay rated for 8A at 250VAC. The contactor coil (an ABB A75-30-00) had a sealed VA rating of 8 VA — well within limits. But the VPR's relay was a standard electromechanical type, not designed for the inrush of a contactor coil (which can be 10-15 times higher at pickup). The relay contacts welding happened within 3 weeks.

The fix: find a VPR with either a solid state output or a relay explicitly rated for contactor coil duty (often listed as "pilot duty"). This is a case where the difference between a relay and a contactor in terms of load type (resistive vs. inductive) matters hugely. Same principle applies to contactors, but VPRs are often treated as a stand-alone component (which, honestly, is a mistake).

7. What's your single most important piece of advice for someone buying or replacing ABB contactors?

Create a pre-order checklist. I know that sounds boring, but hear me out: my 12-point checklist has saved us an estimated $8,000 in potential rework in the last 18 months alone.

The things I verify every single time:

• Coil voltage and type (AC/DC) — confirmed with the customer in writing
• Number of aux contacts (NO vs NC) — get the exact ABB model number.
• Overall dimensions — check the manual for panel clearance.
• Rated current vs motor FLA (always overshoot by 10-15%).
• Rush processing premium — if you're ordering overnight, expect +50-100% (and the budget hit).

In a time-pressured decision (like a breakdown on a Lister Petter diesel generator powered pumping station), you won't have the luxury of rechecking. But if you have a standard part number list from the catalog and you've verified it once, you can trust it under pressure. The time to be careful is before the order goes in.

(A note on honesty: this checklist won't protect you from a shipping error from the supplier. We caught one wrong item on a $3,200 order last month. The packing slip said A26-30-10, the box had A26-30-01. Always visually check. But the checklist does protect you from ordering the wrong thing in the first place.)