Siemens 3VA9988-0AA12 — SENTRON Auxiliary Switch Buying Guide

By Abdullah Zahid
13 min read
Siemens 3VA9988-0AA12 SENTRON Type HQ auxiliary switch changeover contact for 3VA molded case circuit breakers

Siemens 3VA9988-0AA12 SENTRON Auxiliary Switch, Type HQ (7 mm), Changeover Contacts — Specs, Installation, and Selection Guide

If you are specifying or procuring an auxiliary switch for a Siemens 3VA molded case circuit breaker and need to add remote status monitoring, interlock capability, or alarm signaling to your control circuit, the Siemens 3VA9988-0AA12 is one of the most common parts you will encounter in that search. This is a plug-on changeover contact accessory — Type HQ form factor, 7 mm wide — rated at 6 A thermal current, AC 240 V, and DC 250 V, and compatible exclusively with 3VA1, 3VA20, and 3VA25 breaker frames. Confirming those three variables before ordering will save you a return shipment and a project delay.

If you have already verified this is the correct part for your breaker and control circuit, check current pricing and availability for the 3VA9988-0AA12 at LeadTime.ca — ships worldwide.

Who Should Buy the Siemens 3VA9988-0AA12 — and Who Shouldn't

This auxiliary switch is the right choice when all of the following apply to your project:

  • Your main breaker is confirmed as a 3VA1, 3VA20, or 3VA25 series unit — not a 3VA3, 3VA5, or any other 3VA frame
  • Your breaker has the Type HQ mechanical mounting slot (7 mm width) on its top surface
  • Your control circuit operates at AC 240 V or DC 250 V nominal, or any lower AC/DC control voltage within the rated range
  • You specifically need CO (changeover) contact functionality — one normally open and one normally closed contact pair tracking the breaker state
  • A 6 A thermal current rating is adequate for your signal or interlock circuit load
  • Your jurisdiction requires at least one of the following certifications: CE, CCC, EAC, VDE, RCM, or KC

If you need a make-only (NO) contact instead of a changeover contact, look at the 3VA9987-0AA12 or another model from the broader 3VA9 auxiliary switch family. If you are working with a non-Siemens breaker, this accessory will not mount correctly — you need a breaker-specific auxiliary switch from your actual breaker's manufacturer.

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What the Siemens 3VA9988-0AA12 Actually Does in a Control System

The 3VA9988-0AA12 is not a circuit breaker. It does not interrupt, protect, or carry load current in any way. It is an accessory that mounts directly onto a compatible 3VA breaker and provides a changeover contact pair whose state mirrors the breaker's line-side position in real time. When the breaker is closed, the CO contact reports closed. When the breaker trips or is manually opened, the CO contact changes state — the normally open contact opens and the normally closed contact closes, or vice versa depending on how your control logic is wired.

That signal — a clean, electrically isolated contact transition — is what feeds your PLC input module, safety relay, SCADA alarm, or interlock logic. The 3VA9988-0AA12 gives a 3VA breaker a voice in the control circuit. Without an auxiliary switch, a tripped breaker is invisible to automation systems unless someone physically inspects the panel. With the 3VA9988-0AA12 installed, a tripped breaker in a food processing line can instantly trigger an alarm in the SCADA system, initiate an orderly shutdown of downstream VFDs, or disable a dependent production zone — all automatically, before damage or downtime compounds.

The Type HQ designation refers to the 7 mm width form factor of the switch body, which corresponds to the mechanical slot specification on 3VA1, 3VA20, and 3VA25 breaker frames. The switch slides horizontally into that slot and seats with a positive retention. At 56 mm tall and 30 mm deep, the unit is compact enough to fit within standard switchboard and MCC panel clearances without requiring additional mounting space planning.

Where the 3VA9988-0AA12 Sits in a Typical Panel Architecture

This switch occupies the accessory slot on the main breaker and acts as the handoff point between the power distribution circuit and the control signal circuit. It connects the physical state of the breaker to your automation layer without creating any electrical path between them.

  • PLC or DCS controller sends or receives 24 V DC or 120 V AC control signals at the input/output module level
  • Control wiring from the PLC input module runs to the screw terminals of the 3VA9988-0AA12 mounted on the breaker
  • The 3VA9988-0AA12 sits physically on the 3VA1, 3VA20, or 3VA25 main breaker in the distribution panel or MCC bucket
  • The main breaker feeds downstream load: motors, drives, heaters, lighting panels, or sub-distribution boards
  • Breaker state change propagates through the CO contact to the PLC in real time, triggering alarm, interlock, or shutdown logic as configured

Industries and Applications Where This Switch Gets Specified

In food and beverage manufacturing, the 3VA9988-0AA12 is commonly used to detect breaker trips on motor circuits feeding conveyors, mixers, and packaging lines. A trip signal fed back to the SCADA system allows the maintenance team to isolate the fault zone before product spoils or the line backs up.

In automotive OEM facilities and mining operations, auxiliary switches on main feeder breakers are used for interlock sequences. A 3VA breaker protecting a VFD circuit, for example, can use the 3VA9988-0AA12 CO contact to inhibit the VFD's run command the moment the breaker opens — preventing an uncontrolled motor restart when the breaker is manually reset.

Water and wastewater treatment plants use auxiliary contacts for remote monitoring of breakers in unmanned pump stations. The CO contact wired to a remote I/O module allows operators to confirm breaker status from a central control room without dispatching field personnel.

In pharmaceutical and data center environments, where uptime documentation is a compliance requirement, the 3VA9988-0AA12 feeds breaker state into historian software for cycle counting, fault logging, and predictive maintenance scheduling.

Application Typical Deployment
Motor soft-starter or VFD interlocking CO contact inhibits drive run command when breaker opens; prevents uncontrolled restart
PLC-based fault detection and SCADA alarm NO contact wired to PLC input; trip signal triggers alarm or production halt routine
Safety circuit interlock NC contact in safety relay feedback loop; breaker trip de-energizes safety relay output
Multi-zone production flow control Breaker trip in upstream zone disables downstream zone via CO contact and interlock rung
Remote unmanned pump station monitoring CO contact wired to remote I/O; status visible at central control room without field visit
Predictive maintenance and cycle logging NO contact feeds historian or maintenance software to count breaker operations over time

Electrical and Mechanical Specifications That Matter for Ordering

Parameter Value
Model / Catalog Number 3VA9988-0AA12
Contact Configuration CO (Changeover) — one NO and one NC in same switch
Type / Form Factor Type HQ, 7 mm width
Thermal Current 6 A
AC Rated Voltage 240 V (50 Hz / 60 Hz)
DC Rated Voltage 250 V
AC Operational Current (AC-12) 6 A at 12 V through 240 V
DC Operational Current (DC-12) 6 A at 12 V; 4 A at 24 V; 2 A at 48 V; 0.5 A at 110 V; 0.25 A at 250 V
Surge Voltage Resistance 4 kV
Compatible Breaker Frames 3VA1, 3VA20, 3VA25 only

Full technical specifications are available on the product page at LeadTime.ca.

3VA9988-0AA12 vs. 3VA9987-0AA12: Choosing the Right Contact Type

Parameter 3VA9988-0AA12 3VA9987-0AA12
Contact Type CO (Changeover) — NO + NC NO (Make contact only)
Type / Form Factor Type HQ, 7 mm Type HQ, 7 mm
Thermal Current 6 A 6 A
AC Rated Voltage 240 V 240 V
DC Rated Voltage 250 V 250 V
Compatible Frames 3VA1, 3VA20, 3VA25 3VA1, 3VA20, 3VA25
Use Case Redundant monitoring, failsafe interlocks, safety circuits requiring both NO and NC Simple status signal — breaker closed confirmation only

If your control architecture requires both a normally open and normally closed signal — for redundant monitoring or safety-rated interlock logic — the 3VA9988-0AA12 is the correct choice. If you only need a single make contact to confirm breaker closed status, the 3VA9987-0AA12 may be sufficient. Check current availability for the 3VA9988-0AA12 at LeadTime.ca before finalizing your BOM.

Expert Verdict: Is the 3VA9988-0AA12 Worth the Price Over a Generic?

The 3VA9988-0AA12 earns its position in a genuine Siemens 3VA panel because it eliminates the two biggest risks in auxiliary switch selection: mechanical misfit and electrical underperformance. The Type HQ 7 mm form factor is a dimensional specification that must match the breaker's slot geometry exactly. A generic or off-brand auxiliary switch that is nominally described as compatible may not seat fully, creating an intermittent contact that fails under vibration or thermal cycling. The genuine Siemens accessory — factory-tested and matched to the 3VA frame — removes that uncertainty entirely. For a component whose entire job is to give an accurate, repeatable signal, that reliability track record matters more than the unit price delta.

Where the 3VA9988-0AA12 has real limits: it is a standard industrial auxiliary switch, not a functional safety device. If your application requires SIL 2 or PLd compliance, you need to verify whether this switch meets your specific functional safety standard — IEC 61508 or ISO 13849-1 — and supplement it with a safety-rated control module and redundant contact monitoring. The DC current derating is also a legitimate constraint in DC-13 inductive duty applications: at 250 V DC-13, the operational current drops to 0.1 A, which is too low for some relay coil loads. Review the full DC-13 current table in the manufacturer datasheet before specifying this switch in a DC inductive circuit above 110 V.

From a procurement standpoint, the 3VA9988-0AA12 is a stocked accessory in most North American and European distributor warehouses, with standard delivery typically in the 2 to 5 day range for in-stock orders. Lower-demand regions may face 4 to 8 week lead times if the part is not held locally. If you are sourcing for a planned maintenance shutdown or a project build with a fixed commissioning date, confirming live stock before committing to your schedule is the most important procurement step you can take. If this matches your system requirements, view current stock and pricing for the 3VA9988-0AA12 at LeadTime.ca.

For volume pricing on quantities of 10 or more, or to confirm lead time before committing to a build schedule, contact the LeadTime.ca team directly — we ship worldwide.

Wiring and Installation Overview

The following is an overview of key requirements for mounting and wiring the 3VA9988-0AA12. Engineers executing a full installation should refer to the Siemens SENTRON 3VA accessory installation manual for complete procedures and safety instructions.

  • The switch mounts by sliding horizontally into the Type HQ slot on the top surface of a 3VA1, 3VA20, or 3VA25 breaker — always perform this with the main circuit isolated and locked out per your LOTO procedure
  • Confirm full mechanical seating: the switch must sit flush with the breaker top surface with no rotation or lateral shift before wiring begins
  • Control wiring uses screw-type terminals; wire gauge must be 0.5 to 1.5 mm² (16 to 20 AWG per UL) with 15 mm (0.6 in) of insulation stripped from each wire end
  • Tighten all screw terminals to between 0.4 N·m and 0.5 N·m — under-tightening causes contact loosening over time; exceeding 0.5 N·m risks terminal damage
  • Verify NO and NC terminal labeling against your control logic design before energizing the control circuit — reversing the contact wiring will invert your interlock or alarm logic

Commissioning and Functional Verification Overview

  • With main circuit power restored and control circuit energized, use a multimeter to confirm control voltage is present at the auxiliary switch terminals before functional testing
  • Manually trip the main breaker and verify the NO contact opens and the NC contact closes at the receiving device (PLC input, relay, or indicator)
  • Manually reset the main breaker and verify the contact states return to their normal positions
  • Repeat the trip-reset cycle a minimum of five times to confirm consistent, repeatable switching behavior before signing off the installation
  • If connected to a PLC, run a scan diagnostic to confirm the status signal appears correctly on the input module and document the baseline state in your maintenance log

What Engineers Get Wrong When Ordering This Part

Before placing your order for the 3VA9988-0AA12, run through each of the following checks. These are the most common sources of wrong-part returns and installation failures with 3VA auxiliary switches:

  1. Verify your main breaker is model 3VA1, 3VA20, or 3VA25 (NOT 3VA3, 3VA5, or other series)
  2. Confirm your breaker has the mechanical slot for Type HQ (7 mm) auxiliary switch mount
  3. Check that your control voltage is AC 240 V or DC 250 V (verify via nameplate or manual)
  4. Confirm you need CO (changeover) contacts, not NO (make) or NC (break) only
  5. Verify 6 A thermal/operational current is sufficient for your downstream load (signal circuit only)
  6. Check that your region's certifications are present (at minimum: CE for EU/UK, RCM for AU/NZ, CCC for China)
  7. Do NOT confuse this with the main circuit breaker — this is an accessory only
  8. Confirm wire gauges are 0.5–1.5 mm² (16–20 AWG per UL) for screw-type terminals

If every item on that checklist passes, you are ready to order. Confirm availability and pricing for the 3VA9988-0AA12 at LeadTime.ca — or contact our team to discuss volume requirements or expedited sourcing.

Frequently Asked Questions

Can I use the 3VA9988-0AA12 on a 3VA5 breaker?

No. The 3VA9988-0AA12 is specifically designed for 3VA1, 3VA20, and 3VA25 frames only. The 3VA5 frame has a different slot configuration, and attempting to install this switch on a 3VA5 breaker will result in mechanical misalignment and contact failure. Select an auxiliary switch specified for your actual breaker model.

What is the difference between CO (Changeover) and NO (Make) auxiliary contacts?

A CO (Changeover) contact — as in the 3VA9988-0AA12 — provides both a normally open and a normally closed contact pair in one switch, both of which change state when the breaker operates. A NO (Make) contact, such as in the 3VA9987-0AA12, provides only a normally open contact that closes when the breaker closes. Use CO when your application requires redundant monitoring or failsafe interlock logic that needs both contact states; use NO when only a simple closed-state confirmation is needed.

Can I install two 3VA9988-0AA12 switches on the same breaker?

No. The 3VA breaker frame accommodates one auxiliary switch in its single Type HQ slot. If your application requires more contact pairs than one CO switch provides, use the single 3VA9988-0AA12 output to drive an external relay module that provides additional contacts downstream.

At what DC voltage does the 3VA9988-0AA12 current rating drop significantly?

In DC-12 resistive duty, the operational current is 6 A at 12 V, 4 A at 24 V, 2 A at 48 V, 0.5 A at 110 V, and 0.25 A at 250 V. In DC-13 inductive duty, it drops further — to 0.1 A at 250 V DC-13. If your control circuit is a DC inductive load above 48 V, review the full datasheet current table carefully before specifying this switch to confirm the rating is adequate for your relay coil or actuator current draw.

What certifications does the 3VA9988-0AA12 carry, and which ones apply to my region?

The 3VA9988-0AA12 holds CE (EU and UK), CCC (China), EAC (Eurasian Economic Union — Russia, Belarus, Kazakhstan), VDE (Germany), KC (Korea), RCM (Australia and New Zealand), and LR (Lloyds Register maritime). For North American deployments, verify UL 508 and CSA applicability with Siemens or your regional distributor, as regional listing requirements vary by installation type.

How often should I check the screw terminal torque after installation?

Re-torque all screw terminals to between 0.4 N·m and 0.5 N·m at least every six months under normal duty conditions, or annually in high-vibration environments. Wire creep and thermal cycling can cause terminals to loosen over time, resulting in intermittent contact signals. Document the date and findings in your maintenance log each time.

Why Order the 3VA9988-0AA12 From LeadTime.ca

  • Ships worldwide — whether you are sourcing for a facility in Canada, Australia, the Middle East, or Southeast Asia, LeadTime.ca can fulfill your order
  • Specializes in sourcing hard-to-find and distributor-stocked industrial automation and electrical accessories, including the full Siemens SENTRON 3VA accessory range
  • Volume pricing available for orders of 10 or more units — contact the team before submitting a large BOM
  • Fast response to quote requests — useful when your project schedule depends on confirming lead time before committing to an installation date

At-a-Glance Summary

  • The Siemens 3VA9988-0AA12 is a SENTRON auxiliary switch — an accessory only, not a circuit breaker — providing CO (changeover) contact functionality to 3VA molded case circuit breakers
  • Type HQ form factor: 7 mm wide, 56 mm tall, 30 mm deep; slides into the dedicated slot on 3VA1, 3VA20, and 3VA25 breakers
  • Thermal current rating: 6 A; AC rated voltage: 240 V; DC rated voltage: 250 V
  • DC-12 current derates from 6 A at 12 V down to 0.25 A at 250 V; DC-13 inductive derates to 0.1 A at 250 V — verify your duty cycle before ordering
  • Surge voltage resistance: 4 kV; Overvoltage Category III; Pollution Degree 3
  • Screw-type terminals accept 0.5 to 1.5 mm² wire (16 to 20 AWG); tightening torque is 0.4 to 0.5 N·m; strip length 15 mm
  • Certifications: CE, CCC, EAC, VDE, KC, RCM, LR — covers EU, UK, China, Eurasian Economic Union, Germany, Korea, Australia/NZ, and maritime applications
  • Strictly not compatible with 3VA3, 3VA5, 3VA7, or any non-3VA1/3VA20/3VA25 breaker frame
  • Key alternative: 3VA9987-0AA12 for NO (make) contact only; choose based on whether your control logic needs one or both contact states
  • Standard distributor lead time: 2 to 5 days for in-stock regions; 4 to 8 weeks where not locally stocked — confirm before scheduling installation