Build to the rulebook of the exact event and weight class, not to a generic combat-robot recipe. The organizer’s safety officials decide whether your power cutoff, radio failsafe, weapon lock, battery practice, active weapon and restricted systems are legal. Design those safety functions first, document them, and rehearse the inspection demonstration before you commit to motors, batteries or armor.
The National Havoc Robot League (NHRL) Open Rules for 2026 provide a useful current example, but they are not universal. Other leagues can set different voltage limits, dimensions, lock requirements, testing procedures and approval deadlines. Treat the NHRL details below as a concrete model, then replace them with the current documents and instructions from your target organizer.
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Start with the sanctioned event and weight class
A combat robot is safe and legal only in relation to a particular competition. A design that passes one arena’s inspection can fail another because the cutoff location, weapon definition, battery policy or class limit differs.
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Choose the class before choosing components
Record the event, class, maximum mass, dimensions, voltage limits, weapon rules, radio requirements, testing area and inspection schedule. Do this before selecting a motor, battery pack, speed controller or weapon diameter. The class determines how much mass remains for armor, guards, wiring and safety hardware after the drive and weapon systems are installed.
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Use the rulebook and safety staff as the authority
Download the current rulebook, construction specifications and arena-safety guidance for the event. Ask safety staff about an unusual mechanism before buying parts. Written approval is more valuable than an assumption that an inspector will accept a finished robot at check-in.
- Save the rule version and publication date used for your design.
- Mark every requirement that needs a physical demonstration.
- Keep written answers from officials with your build notes.
- Recheck the documents shortly before travel; class limits, voltage rules and approval deadlines can change.
Design safety as a system
Separate the robot into four safety layers: an external power cutoff, a radio-loss failsafe, a physical weapon lock and controlled testing procedures. None substitutes for another. A removable link that cuts drive power does not prove that a weapon controller stops on signal loss, and a software stop does not make a charged weapon safe to handle.
Make every safety control visible and reachable
Place the cutoff, status indicators and lock points where an inspector can identify them without removing armor. Label them with durable markings. Route wires so a person can reach the control without placing a hand near a blade, drum, saw or pinch point.
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Inspection must cover the robot as it will compete: installed batteries, final armor, weapon, receiver, covers and removable links. A failsafe test on a bench receiver or a lock tested before the battery is installed does not establish safe behavior in the final assembly.
Provide a positive power cutoff
The cutoff must remove power from both drive and weapon systems. Depending on the event, it may be a switch, removable link or another approved device. It must be accessible without disassembling the robot, have adequate strain relief where required, and be operable quickly by a person standing clear of hazards.
NHRL 2026 example
Under the NHRL 2026 Open Rules, the cutoff must be deactivated within 15 seconds. For 12 lb and 30 lb robots, it must be reachable while the robot is in its primary fighting orientation and use a dedicated removable link with adequate strain relief. A 3 lb robot may use battery connectors as removable links. NHRL states that the cutoff must be operable with one hand, although competitors may use both hands at the event.
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Those figures are NHRL pass criteria, not universal engineering targets. If another organizer specifies a different time, orientation or connector arrangement, that rule controls.
Build and test the cutoff
- Identify every circuit that can move the robot or weapon, including auxiliary actuators that could create a hazard.
- Place the switch or link where an official can reach it from the required orientation.
- Add strain relief so pulling the link cannot tear a conductor from a terminal or expose energized metal.
- Mark the control and provide a safe way to remove it without touching the weapon.
- With the robot secured, activate drive and weapon systems, remove or open the cutoff, and verify that both lose power.
- Repeat the test with the final battery, armor and radio installation.
Engineer the radio-loss failsafe
A failsafe is the robot’s response when the transmitter is switched off, the receiver loses signal or the control link is interrupted. Drive should stop and the weapon should stop within the event’s specified windows. Test signal loss deliberately; do not rely on a transmitter’s normal neutral position.
NHRL 2026 example
NHRL’s safety inspector oversees a test in which drive must stop within 30 seconds of signal loss or transmitter power-down, and weapons must stop within 60 seconds. Autonomous robots still need radio control for remote activation and deactivation. These are NHRL’s criteria for its 2026 rules, not a general industry standard.
Verify each channel independently
- With the weapon disabled by its physical lock, confirm that loss of signal commands drive to a stationary state.
- With the robot restrained in the approved test area, confirm that signal loss stops the weapon within the required window.
- Check that reconnecting the transmitter does not start a motor or weapon automatically.
- Test low battery, receiver reboot and transmitter power-down scenarios if the event’s officials request them.
- Record the test setup and result so another team member can repeat it.
Configure the receiver and motor controllers so a lost signal produces a defined stop rather than holding the last command. If a controller cannot provide the required behavior, replace it or obtain written approval for an alternate architecture before competing.
Use a physical weapon lock whenever the robot is handled
A weapon lock must prevent actuation, not merely make movement inconvenient. Use a robust pin, bar, block or event-approved restraint that can withstand an accidental command and is visually obvious to an inspector. A software disable, transmitter switch or unplugged signal lead is not a substitute unless the rulebook explicitly says so.
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NHRL sets different permitted movement for rotating kinetic weapons and non-kinetic weapons and requires special arrangements for some designs. Build the lock around the rule’s allowed movement, not around a generic “locked” label. If a rotating weapon can coast, design the restraint so the remaining motion cannot reach a person or release the lock.
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Install and remove the lock under a written procedure
Keep the lock installed whenever the weapon system and batteries are installed, removing it only immediately before a match or when an official directs you to do so under the event’s procedure. Cover sharp edges that could cause serious cuts or punctures while the robot is moved around the venue. Store the lock on the robot or in a marked team kit so it cannot be misplaced.
Test only in the event’s controlled area
NHRL requires weapon and drive testing in a test box. Its wheels-up exception applies only when the robot has a second dedicated switch for its weapon system. A workbench, pit floor or home garage is not an informal replacement for the event’s test area.
Safe test sequence
- Confirm the weapon lock, sharp-edge covers and power cutoff are installed.
- Clear people, loose tools and conductive debris from the area.
- Place the robot in the designated test box or other organizer-approved enclosure.
- Have one person operate the transmitter and another person act as a safety observer.
- Apply power, verify the cutoff is reachable, and test controlled movement at the direction of event staff.
- Remove the weapon lock only when the official procedure allows it; reinstall it immediately after testing.
- Remove power before changing wiring, firmware, radio settings or mechanical parts.
Handle batteries and workshop tools as hazards
Battery safety is part of the competition rules, not merely a charging preference. Lithium-chemistry packs can deliver enough current to melt wiring, ignite damaged cells or drive an unsecured weapon. Use the chemistry, connector, fuse and charger arrangement permitted by the event.
NHRL 2026 battery requirements
NHRL requires charging to be attended and requires lithium-chemistry batteries to be charged using balance leads. Do not leave a charging pack unattended where the rules prohibit it. Stay with the pack, monitor for swelling, heat, odor or damaged insulation, and stop charging if anything changes.
- Inspect cells, leads, connectors and insulation before charging.
- Use a charger configured for the pack’s chemistry and cell count.
- Keep the pack away from metal scraps, flammable materials and moving mechanisms.
- Disconnect the battery before servicing the robot.
- Transport and store packs according to the venue’s written policy.
Wear appropriate workshop PPE
NHRL requires safety glasses and other appropriate personal protective equipment when using power tools in the pits or machine shop. Impact-rated safety glasses are a sensible baseline, but the venue’s tool policy determines what is required. Add hearing, respiratory, hand or face protection when the tool and material call for it. Never hold a small part in your hand while drilling, grinding or cutting.
Get approval for restricted systems early
Some mechanisms need a design review before arrival. NHRL identifies pneumatic and hydraulic systems, combustion engines, flame systems and exposed moving components above its stated speed threshold as restricted examples. Requirements and lead times depend on the system.
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Submit a complete approval package
Contact safety staff while the design is still changeable. Describe the energy source, pressure or fuel, ignition method, reservoirs or tanks, valves, containment, weapon lock, emergency shutdown and test plan. For a flame design, NHRL’s approval guidance asks for details such as fuel, ignition, weapon safety and tank protection.
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- Show where the operator and official can shut it down.
- Explain how the system remains safe after radio loss.
- Provide drawings or photographs of containment and protection.
- Ask for the approval deadline and retain the written decision.
Do not finish a restricted design on the assumption that inspection will approve it on arrival. If approval is denied or conditions change, have a legal fallback configuration.
Make the active weapon legal and demonstrable
Safety does not excuse an ineligible weapon. NHRL’s 2026 rules require an active weapon designed to visibly and consistently harm an opposing robot. The 2026 Rules and Judging Criteria Q&A states: “This doesn’t require weapons to be capable of destruction, but it does require intentional, effective use of the active weapon that makes the robot legal at NHRL.”
Design the weapon so it can be locked, tested in the approved enclosure and operated intentionally during a match. A mechanism that is technically present but cannot be used meaningfully, safely or consistently can create both a legality problem and an inspection problem.
Compare design options by safety and inspectability
Because the event, class and robot concept are unspecified, no particular motor, battery, chassis material or weapon architecture can be ranked responsibly. Use these decision axes for any options you are considering:
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|---|---|
| Legality | Does the current rulebook allow the mechanism, energy source, dimensions, mass and active-weapon behavior? |
| Lock and cutoff | Can a physical lock prevent actuation, and can one accessible control remove power from every hazardous system? |
| Failsafe proof | Can you demonstrate the required stop behavior after signal loss with the final wiring and battery installed? |
| Complexity and inspection | Can an official understand, reach and test the safety controls without disassembling the robot? |
| Mass and class allowance | After batteries, armor, guards and safety hardware, does the complete robot remain within the class limit? |
| Approval burden | Does the design use pneumatics, hydraulics, combustion, flame or high-speed exposed parts that require advance approval? |
Build in an inspection-first sequence
- Freeze the rule set. Name the organizer, class, event date and rule version in your project notes.
- Draw the energy map. Trace battery power through cutoff devices, controllers, motors, weapon systems and auxiliary actuators.
- Place safety hardware. Reserve physical space for the removable link or switch, lock, strain relief, labels and sharp-edge covers before packaging performance parts.
- Implement failsafe behavior. Configure the receiver and controllers so signal loss commands a stop and reconnect cannot cause an automatic start.
- Build a restrained prototype. Test each circuit with the weapon physically prevented from moving and with the battery disconnected between changes.
- Integrate the final robot. Repeat cutoff, failsafe, lock and edge-cover checks after armor, wiring and batteries are installed.
- Document restricted systems. Submit approval materials and wait for written guidance before final fabrication.
- Rehearse inspection. Have a teammate call out each requirement while you demonstrate it in the same orientation and configuration used at the event.
Prepare for the safety inspection
NHRL inspection includes weapon locks or covers, sharp-edge covers, radio failsafe behavior, controlled movement, power cutoff operation, active-weapon functionality and other design-specific requirements. Organize the robot and team so every test can be performed without improvisation.
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Inspection-day checklist
- Current rulebook and any written design approvals.
- Installed, labeled cutoff or removable link with required strain relief.
- Weapon lock and any required secondary weapon switch.
- Sharp-edge covers for transport.
- Charged batteries, balance leads and approved charger arrangements.
- Transmitter, receiver and spare control hardware configured for the robot.
- Documentation showing failsafe and cutoff test results.
- Required PPE for pits, machine shop and controlled testing.
- A team member who knows how to power down, lock and carry the robot.
Demonstrate, do not merely describe
Show the official the actual cutoff, remove power, apply the weapon lock and perform the radio-loss test when requested. Keep hands away from the weapon path and follow the official’s instructions about when a lock may be removed. If a test fails, stop, reinstall the lock, remove power and ask what correction and retest the official requires.
Troubleshoot common safety failures
| Symptom | Likely cause | Safe response |
|---|---|---|
| Cutoff cannot be reached in the fighting orientation | Control is behind armor or on the wrong face | Do not compete with the current layout; relocate the control or obtain written approval for an alternate arrangement. |
| Removing the link stops drive but not the weapon | Weapon power bypasses the required cutoff | Remove the battery, trace every power path and redesign the cutoff so all hazardous systems lose power. |
| Robot keeps moving after transmitter shutdown | Receiver or controller holds the last command | Change the failsafe configuration or hardware, then retest under the event’s timing. |
| Weapon starts when the transmitter reconnects | Controller restores the previous command or an arming sequence is incomplete | Disable automatic restart, add an explicit arming step if allowed, and test with the weapon restrained. |
| Lock cannot be inserted with the battery installed | Lock path was added after the mechanism | Redesign the lock and access point; do not substitute a software setting. |
| Battery becomes hot, swollen or damaged while charging | Cell, charger, wiring or balance connection fault | Stop charging, isolate the pack according to venue procedure and have qualified safety staff assess it. |
| Restricted-system approval is missing | Design was treated as an inspection-day decision | Contact the organizer before travel and prepare a legal non-restricted configuration. |
Know which design details remain conditional
No responsible component-level build plan can be selected from the title alone. Chassis material, motor size, battery voltage and capacity, weapon dimensions, armor thickness, connector ratings and controller choices depend on the event, class, target performance, budget, fabrication capability and physical layout. Supplying those details without those inputs would be guesswork and could produce an unsafe or illegal robot.
Once those inputs are fixed, evaluate every part against the same questions: does it fit the class, can it be isolated by the cutoff, does signal loss stop it within the rule’s window, can it be physically locked, and can an inspector verify those facts without hazardous handling?
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Operate safely between matches
Keep the weapon locked whenever the robot is transported, stored or worked on. Remove power before opening armor or changing wiring. Assign one person to control the transmitter and another to carry or steady the robot. Announce when the lock is being removed, clear the area, and follow the event’s official sequence for arming and testing. After a match, reinstall the lock before the robot leaves the arena-side area.
These habits protect the team even when a rule does not spell out every workshop scenario. They also make inspection faster because the robot arrives in a known, controlled state.
The Bottom Line
A safe sanctioned-competition robot begins with the target event’s current rulebook: add an accessible full-system cutoff, prove radio-loss stops, use a real weapon lock, charge and test under venue procedures, and obtain advance approval for restricted systems. The NHRL 2026 timings and hardware examples show how specific those requirements can be; your organizer’s officials remain the final authority.
Quick Recap
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