Battery Disconnect Switch Auxiliary Contacts, Interlocks and State Feedback: RFQ Evidence Guide

A remote lamp marked BATTERY OFF can be illuminated while the main current path is still energized. A handle can point to OFF while an external charging or bypass path remains live. An auxiliary contact can change state before, after or independently of the power contacts. These are different facts, and a procurement specification must not collapse them into one “status” signal.

For an OEM buyer, the direct answer is: define the commanded state, actuator or handle state, auxiliary-contact state, verified main-contact state and downstream system voltage as separate RFQ fields. Require a model-specific contact diagram, switching sequence, ratings for the auxiliary circuit, failure-mode boundaries and a project truth table. Approve an indicator or interlock only for the claim that its evidence actually supports.

This guide covers manual and remotely operated low-voltage battery disconnects used as components in vehicles, equipment, marine systems and similar DC architectures. It complements the battery selector and dual-circuit state guide, the battery disconnect specification guide and the contact-resistance and temperature-rise guide. Those guides address topology, duty and current-path performance. Here the narrower purchasing question is what an auxiliary signal means and how that meaning is proved.

Nothing in this article confirms a SINAWATTS switch configuration, auxiliary-contact arrangement, functional-safety level, certification, rating, test capability, inventory, price, MOQ, lead time or customer result. Every value and sequence must be confirmed for the exact offered part and system.

Direct answer: which states must the buyer keep separate?

At minimum, name five state layers on the system drawing:

1. Command state — what the controller, remote rocker, key switch or operator requested.
2. Actuator or mechanism state — where the handle, plunger, latch or internal mechanism is reported to be.
3. Auxiliary-contact state — whether each low-current NO, NC or changeover contact is electrically open or closed at its defined terminals.
4. Main-contact state — whether the power poles are electrically conducting, open, welded, high resistance or otherwise outside the expected condition.
5. System-energy state — the voltage or stored energy that remains at the protected load, bus or enclosure after all intended paths and discharge behavior are considered.

A sixth layer is often useful: permission state, the result produced by control logic after it evaluates feedback, timing and other prerequisites. Permission might allow an engine stop sequence, charger disable, service access or contactor operation. It is a logic decision, not a physical contact.

The RFQ should state which layer each signal represents. “ON LED,” “OFF feedback,” “auxiliary output,” “interlock” and “isolation status” are not sufficiently precise without that mapping.

An auxiliary contact can serve different functions

An auxiliary contact is a lower-current switching element associated with a main switching device. Depending on the exact construction, it may be used for indication, sequencing, controller input, alternator-field control or another interlock function. Its purpose does not follow from terminal size or the word “aux.”

IEC’s official record for IEC 60947-5-1:2024, checked on 2026-10-04, states that the standard applies to control-circuit devices and switching elements used for controlling, signalling and interlocking, up to the scope limits stated there. That public scope provides useful terminology for a control circuit. It does not show that a particular automotive or marine battery switch complies, that its auxiliary contact is safety rated, or that the contact mirrors the main power path.

The main switching function is a separate subject. The official record for IEC 60947-3:2020 with Amendment 1:2025 covers specified switches, disconnectors, switch-disconnectors and fuse-combination units within its scope. A supplier should identify the standard, edition, device classification and exact model evidence it claims. A buyer should not combine the public scopes of IEC 60947-3 and IEC 60947-5-1 into an unverified compliance claim.

Use one of these functional labels for every auxiliary circuit in the RFQ:

• indication only: drives or informs a lamp, display or telemetry input;
• controller feedback: reports a defined mechanism or contact condition to logic;
• sequence contact: changes before or after the main contacts to create an ordered shutdown or startup;
• permissive interlock: must be in the required state before another action is allowed;
• inhibit contact: blocks another source, actuator or operating mode;
• diagnostic channel: supports discrepancy detection when compared with command or another independent signal; or
• service/lockout indication: reports a defined mechanical lock condition, with separately documented isolation procedure.

The same physical contact may support more than one function only if its ratings, logic, independence and failure analysis support all of them. A spare terminal should not be assigned a safety or shutdown role by assumption.

Read the switching sequence before assigning the signal

Contact timing can be deliberately offset. That offset can be the main value of the auxiliary circuit.

Littelfuse’s official drawing for the named 75930 Series 75930-02, checked on 2026-10-04, states that the main contact turns on before the auxiliary contact and that the main contact turns off after the auxiliary contact. The drawing separately lists main and auxiliary terminals and their model-specific ratings. This sequence can support a controlled circuit action before the high-current path opens. It also proves why an auxiliary transition should not be interpreted as simultaneous proof that the main path has opened.

The corresponding Littelfuse 75930-02 product record identifies the exact switch as a manual battery master disconnect with an auxiliary contact and publishes separate main- and auxiliary-circuit data. Those values belong only to the named configuration. They are evidence categories to request, not generic values for another battery disconnect.

TE Connectivity’s official KISSLING Series 35H high-voltage battery disconnector datasheet, checked on 2026-10-04, gives another model-specific sequence: during OFF-to-ON operation, its main contacts close before the auxiliary contact; during ON-to-OFF operation, the auxiliary contact opens before the main contact. It also states that switching the main path under load is not permitted for the named device. This is a useful example of a sequenced auxiliary circuit, but it does not establish timing, voltage class or operating permission for another series.

These sources lead to a procurement rule: request the contact sequence in both directions. A statement such as “auxiliary contact included” does not reveal whether it is early-break, late-make, simultaneous within a tolerance, mechanically independent, or simply connected to a control module.

Command feedback and main-contact proof are not the same

A remotely operated disconnect adds another gap between request and physical result. A coil command may be accepted while the mechanism stalls. A latching switch may retain its last position after control power is removed. A manual override may place the device in a state that differs from the last electrical command.

Blue Sea Systems’ official ML-RBS 7700 product record, checked on 2026-10-04, identifies an LED output to indicate switch state and a manual override that permits control without power and offers a locked-off service position. Those are attributes of that named product. The record does not authorize a buyer to label every remote switch output as direct proof of galvanic isolation, nor does it define another manufacturer’s manual-override behavior.

For any remote device, ask these questions:

• Is the returned signal derived from the command input, coil drive, electronic controller, mechanical linkage, auxiliary contact or a direct main-path sensor?
• Does the signal change when the manual override is used?
• What happens after loss and restoration of control power?
• Is the device monostable, bistable or magnetically latched?
• Can a jammed actuator leave command and feedback disagreeing?
• Can welded or contaminated main contacts leave auxiliary feedback in the expected state?
• Is feedback valid during motion, or only after a documented settling interval?
• Does the signal have a defined unknown or fault state, or only ON and OFF?
• Which terminal, reference and polarity define the output?
• What current must flow through the feedback circuit for reliable operation?

If the supplier cannot describe the signal source, call it unverified indication, not main-contact proof.

Build a project truth table before requesting quotations

Give every signal a stable name and active convention. Avoid a column simply marked “1,” because 1 could mean coil energized, contact closed, bus live or logical permission granted.

On small screens, swipe or scroll sideways to read every column. Keyboard users can focus the table and use the arrow keys.

This is a template, not a universal operating sequence. Replace each cell with the project requirement and supplier’s exact response. Include precharge circuits, alternators, chargers, solar controllers, DC-to-DC equipment and unswitched emergency feeds where applicable. A switch can perform exactly as designed while the system bus remains energized through another route.

Specify the auxiliary electrical load, including minimum load

An auxiliary contact needs its own electrical specification. A contact suitable for one lamp circuit may not be suitable for a high-impedance digital input, and a logic input may not provide enough wetting current to maintain a stable low-level contact interface. Conversely, a contact or output intended only for logic may not be permitted to drive a lamp or relay directly.

Require the supplier to return:

• contact form: NO, NC, changeover or another documented arrangement;
• terminal numbering and diagram viewing direction;
• rated operational voltage and current for the applicable DC load type;
• permitted resistive, inductive or lamp load, where differentiated;
• minimum switching voltage and current, if specified;
• maximum inrush or transient condition, if specified;
• contact material or output technology only when documented;
• isolation between auxiliary, control and main circuits;
• leakage current and voltage drop for a solid-state output;
• debounce, transition or propagation characteristics;
• required suppression for an inductive load and its effect on release time;
• expected state with control power absent; and
• connector, terminal, wire-size and sealing requirements.

Do not infer an auxiliary rating from the main-contact rating. The Littelfuse 75930 documents are useful precisely because they publish the circuits separately. The buyer must obtain the same separation for the offered model.

Illustrative logic-input check only

Assume an invented controller provides 5 V through a 10 kΩ input resistance to a dry auxiliary contact. The closed-contact current would be approximately:

I = V / R = 5 V / 10,000 Ω = 0.0005 A = 0.5 mA

Now assume, only for the exercise, that a proposed contact document specifies a 10 mA minimum switching current. The 0.5 mA interface would not meet that stated minimum. Increasing the software debounce time would not fix the electrical mismatch. The buyer would need a supported interface, such as an approved input circuit, an interposing device or a different feedback option.

The values are fictional and do not describe a SINAWATTS, Littelfuse, TE or Blue Sea product. The calculation demonstrates why both maximum and minimum auxiliary loads belong in the RFQ.

Define sequencing and discrepancy time without hiding uncertainty

Sequence requirements should be stated as intervals and conditions, not adjectives such as “fast” or “before.” Identify the event that starts timing and the electrical or mechanical event that stops it. Include tolerances, temperature, supply-voltage range, initial state and repeated-operation conditions.

For a remote disconnect, useful timing fields include:

• input-command recognition;
• coil or motor energization;
• auxiliary-contact transition;
• main-contact make or break;
• contact bounce or feedback stabilization;
• bus-voltage decay, if separately measured;
• controller debounce;
• discrepancy timeout; and
• re-command lockout or recovery time.

Illustrative timeout calculation only

Suppose a fictional project’s approved evidence gives a maximum 180 ms from valid OFF command to stable auxiliary OFF feedback over its specified voltage and temperature range. The controller applies 40 ms of input/output debounce, and the responsible engineer assigns 80 ms for communication and measurement uncertainty. A draft diagnostic timeout could be checked arithmetically as:

180 ms + 40 ms + 80 ms = 300 ms

In that fictional design, feedback arriving at 240 ms is inside the 300 ms diagnostic window, while feedback still absent after 300 ms creates a discrepancy. This arithmetic does not prove the main contacts are open, define a safe discharge time, or provide a universal safety margin. It only shows how a documented device interval can be kept distinct from controller and measurement allowances.

If the project needs confirmed isolation before service access, add the approved absence-of-voltage or energy-control procedure. A timer and auxiliary bit alone should not be relabelled as that procedure.

Ask what physical feature operates the auxiliary contact

The strength of a feedback claim depends on the physical linkage and defined failure modes. Request a section, mechanism description or manufacturer statement that explains what moves the auxiliary element. Useful distinctions include:

• operated by the external handle;
• operated by the same cam as the main contacts;
• operated by the moving bridge or armature;
• derived electronically from coil current or actuator position;
• derived from measured main-path voltage or current; or
• generated by a controller that estimates state.

Then ask whether the construction has any documented direct-opening, mechanically linked, mirror-contact or other defined behavior under the claimed standard. These terms have specific scopes. Do not apply one because the drawing merely shows contacts on the same shaft.

A contact mechanically linked to the actuator can provide stronger position evidence than a repeated command bit, but it can still have limitations. Linkage tolerance can create a transition zone. Contact contamination can make the low-current signal intermittent. A broken lead can look like an open contact unless the diagnostic circuit distinguishes it. A single auxiliary pole may not reveal disagreement between multiple main poles.

Require the supplier’s declared failure behavior rather than inventing it from the schematic. If the project has a safety-related function, the responsible functional-safety process must establish architecture, diagnostic coverage, independence, common-cause controls, validation and required performance. A standard auxiliary contact is not automatically a safety channel.

Treat alternator or source interlocks as an ordered system function

Some auxiliary contacts are intended to unload or disable a source before the battery path opens. The early-break/late-make sequences in the Littelfuse 75930 and TE KISSLING 35H records show why order matters. They do not define the correct interface for every alternator, charger, inverter or engine controller.

For a source-disable interlock, the RFQ should include:

1. the source manufacturer and exact control terminal or permitted shutdown input;
2. the source’s voltage, current and suppression requirements at that interface;
3. the required order of auxiliary and main-contact events;
4. the minimum time between source-disable recognition and main-path opening;
5. behavior if the engine/controller does not acknowledge shutdown;
6. behavior during a broken auxiliary wire or short to supply/ground;
7. manual-override behavior;
8. restart and reconnection conditions; and
9. evidence that the complete sequence was validated for the intended system.

Do not connect an alternator field, ignition or controller line to an auxiliary terminal solely because the current seems low. The equipment manufacturer must permit the interface. Stored energy, controller logic and suppression can affect both the source and the contact.

The battery switch cranking-duty guide explains why carrying current and switching current remain separate. An auxiliary sequence cannot expand the main device’s load-breaking permission.

Request evidence at three levels

Separate component evidence, system validation and production/installation records.

Component evidence

This should identify the exact switch, contact diagram, terminal designations, sequence, tolerances, auxiliary ratings, environmental scope, durability evidence and operating restrictions. Request the current drawing and instruction revision rather than a family brochure.

System validation

This should show how the exact switch interfaces with the controller, source, load and external energy paths. Capture command/feedback timing, debounce, fault injection where approved, manual override, power loss, bus behavior and recovery. A component report cannot know every external bypass or software state.

Production and installation records

These should verify part identity, wiring, pinout, connector seating, parameter/software revision, labels and an approved functional check. An end-of-line lamp check may show that one signal changes; it does not recreate endurance, contact welding or system fault validation.

Use the harness pinout and cavity-view guide to control feedback-wire identity and the connector terminal-retention guide to keep seating evidence separate from logical function.

RFQ evidence-request matrix

On small screens, swipe or scroll sideways to read every column. Keyboard users can focus the table and use the arrow keys.

The hold column should remain visible in sourcing reviews. Missing evidence is not proof of failure, but it is not approval either.

Compare three illustrative supplier returns

The following comparison is fictional. It is not a customer case or measured product result.

Offer A lists “remote battery isolator with LED feedback.” The diagram shows a controller output, but it does not say whether that output reflects the command, actuator or main contact. No minimum logic load, manual-override behavior or fault state is provided. The correct disposition is hold feedback approval while the basic power-switch offer is reviewed separately.

Offer B supplies a mechanical auxiliary contact and a drawing that states the auxiliary contact opens before the main contact. The ratings fit the proposed controller input, and the manual override moves the same mechanism. However, the bidder calls the early auxiliary opening “proof of isolation.” That claim exceeds the evidence. The signal may support shutdown sequencing, but main-path and system-energy verification remain separate.

Offer C supplies the exact contact sequence, auxiliary ratings, mechanism description, manual/power-loss table and a system-validation report covering the offered controller and wiring. The report records command, feedback, main-path measurement and bus voltage on a common timeline across the required conditions. Offer C is reviewable, not automatically acceptable: the responsible engineer must still compare its samples, faults, environmental range and acceptance limits with the project.

This comparison prevents a richly labelled indicator from outranking a less polished but better bounded evidence package.

Follow a seven-step procurement decision

1. Freeze the system topology

Name batteries, loads, sources, disconnect poles, bypasses, precharge paths, grounds/returns and stored-energy elements. Define exactly what “disconnected” is intended to mean.

2. Freeze the operating sequence

Describe normal connect/disconnect, emergency command, service lockout, manual override, control-power loss and restart. Identify actions prohibited under load.

3. Assign every signal to a state layer

Mark each input/output as command, mechanism, auxiliary contact, main path, bus measurement or calculated permission. Remove ambiguous “status” labels.

4. Match the auxiliary electrical interface

Check contact form, minimum and maximum load, DC load type, polarity, suppression, leakage, debounce, connector and wire. Resolve incompatibility before samples.

5. Approve the evidence and test plan

Map component documents to the exact model. Define sample count, supply and temperature conditions, event timestamps, failure cases, acceptance criteria and raw-data retention.

6. Validate the complete state machine

Evaluate normal transitions and approved fault cases, including manual operation, broken feedback wiring, command disagreement and external energization. Use qualified personnel and a controlled test setup; do not improvise tests on an exposed live battery system.

7. Release production and change controls

Freeze device, harness, controller hardware, software/calibration, timing parameters and labels. Set end-of-line checks, discrepancy reaction, record retention and revalidation triggers.

Control changes that can invalidate state evidence

Require advance review before changing:

• switch manufacturer, series, part number, suffix, pole count or contact option;
• cam, linkage, spring, actuator, manual override or lockout construction;
• auxiliary-contact material, form, rating, terminal or connector;
• main-contact construction where it affects sequence or state correlation;
• coil voltage, control module, latching behavior or power-loss state;
• harness pinout, wire, splice, ground/reference or shielding;
• input pull-up/pull-down, wetting circuit, suppression or debounce;
• controller hardware, firmware, calibration or diagnostic timeout;
• alternator, charger, inverter, precharge or bypass architecture;
• mounting orientation or environmental sealing where allowed operation can change;
• end-of-line functional-test method, fixture or software; and
• claimed standard, certificate, report or instruction revision.

A firmware-only change can alter the meaning of a perfectly functioning auxiliary contact. A mechanical-only switch substitution can invalidate a software timeout. Review both sides of the interface.

Send a complete auxiliary-contact and feedback RFQ

Provide the one-line diagram, state truth table, operating sequence, current duties, auxiliary electrical interface, controller logic, manual-override requirement, environmental range, diagnostic response, qualification plan and production-record requirements. Ask bidders to return an exact model, controlled diagrams, sequence data, ratings, physical feedback source, limitations, deviations and change-notification list.

Keep commercial terms separate and current. Request the price, sample terms, MOQ and lead time in writing for the accepted configuration; no value can be inferred from an auxiliary-contact option or from this guide.

Send a battery disconnect auxiliary-contact and state-feedback RFQ with the truth table and system drawing. Ask for written confirmation of what every returned signal proves, what it does not prove, and which exact device documents support that boundary.

Buyer FAQ

Does an OFF auxiliary contact prove that the main battery path is open?

Not automatically. It proves only the state defined for that contact and construction. Request the physical linkage, switching sequence, failure-mode boundaries and model-specific validation. Confirm system energy through the approved procedure, including external paths.

Is an LED marked ON/OFF direct contact feedback?

Only if the manufacturer’s documentation says how the LED output is derived and what it represents. It may reflect a command, controller state, mechanism position or auxiliary contact. Treat it as unverified indication until the source and limitations are documented.

Why would the auxiliary contact open before the main contact?

An early-break contact can initiate a source-disable or controller sequence before the power path opens. Littelfuse 75930 and TE KISSLING 35H documents show product-specific ordered behavior. The actual application interface and timing still need validation.

Can an auxiliary contact switch an alternator field or ignition circuit directly?

Do not assume it can. Obtain the source manufacturer’s permitted interface and electrical characteristics, then compare them with the auxiliary contact’s DC load rating, minimum load, transient behavior and suppression instructions.

What is the difference between NO and NC in an RFQ?

Define the reference condition. “Normally” may refer to the unoperated mechanism, de-energized coil or another manufacturer-defined state. Include a terminal diagram and state table so the label cannot be misread.

Should feedback be fail-safe?

The project’s risk and functional-safety process must define the required failure response, diagnostics and architecture. A single NC loop may reveal one open-wire fault, but it does not by itself establish a safety performance level or detect every welded-contact, short-circuit or common-cause fault.

Can a software delay replace missing contact-sequence data?

No. A delay can be part of an approved control strategy, but it does not reveal when the physical contacts change or guarantee the source has shut down. Base timing on applicable device and system evidence.

How should a manual override appear in the truth table?

Give it separate rows for each permitted manual position, control-power condition and feedback result. State whether remote commands are inhibited and how the controller detects or records the override.

What if a switch has two main poles but one auxiliary contact?

Ask what the single contact follows and whether it can detect disagreement between the main poles. Do not assume one auxiliary element proves both current paths. The project may need additional sensing or a different device architecture.

Can a voltage sensor replace an auxiliary contact?

It answers a different question. A voltage measurement can show a bus condition within its measurement boundary, while an auxiliary contact can report mechanism or sequence state. Some systems use both so command, device position and residual energy can be compared.

What production check is useful?

Verify exact part identity and pinout, then exercise the approved states in a controlled fixture while recording command and feedback. Add the checks required by the design for the main path and external bus. A lamp that changes color is not a complete validation record.

Does this guide confirm that SINAWATTS supplies a switch with auxiliary feedback?

No. It is an RFQ framework. Ask for the current offered part number, diagrams, ratings, documents, commercial terms and project-specific evidence before selection.

Official sources checked on 2026-10-04

• IEC 60947-5-1:2024 official record — control-circuit device, signalling and interlocking scope.
• IEC 60947-3:2020 with Amendment 1:2025 official record — main switching-device scope and edition reference.
• Littelfuse 75930-02 product record and official 2D drawing — exact-model auxiliary circuit and ordered contact behavior.
• TE Connectivity KISSLING Series 35H datasheet — exact-series auxiliary sequence, ratings and operating boundary.
• Blue Sea Systems ML-RBS 7700 product record — exact-model LED state output and manual-override description.

These pages were used within their stated scopes. None establishes compatibility, compliance or capability for an unnamed or SINAWATTS-branded product.