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Combat robots need four core protections before they can compete safely: a reachable master power cutoff, a tested radio failsafe, a positive weapon lock, and covers for hazardous edges during handling. Competition rules also govern battery voltage and charging, restricted weapon systems, testing areas, protective equipment, and inspection. The exact limits belong to the event and robot class; NHRL’s 2026 Open Rules provide a detailed example, not a universal standard.
Start with the event’s current rulebook
There is no single worldwide safety specification for combat robots. Requirements change with the organizer, weight class, weapon type, power system, and arena. A robot that passes one event can require redesign or written approval at another. Before choosing motors, batteries, pneumatics, or a weapon, download the current rules for the exact event and class and check the effective version.
NHRL’s 2026 Open Rules are a useful concrete reference. They define the cutoff, failsafe, weapon-lock, battery, testing, and inspection controls described below. Their timings and class-specific details must not be presented as universal. The official rules page for another major league lists Design Rules and Tournament Rules version 2027.01, last updated October 1, 2026, and also notes that a 2027 draft is available. That illustrates why checking the version and effective date matters.
The four safety features inspectors expect first
| Feature | NHRL 2026 requirement | What the builder should demonstrate |
|---|---|---|
| Master power cutoff | Removes power from drive and weapon systems; accessible without disassembly; actuated with one hand; deactivated within 15 seconds | An inspector can reach it in the primary fighting orientation for 12 lb and 30 lb robots and can see that both drive and weapon power are removed |
| Radio failsafe | After signal loss or transmitter shutdown, drive stops within 30 seconds and weapons within 60 seconds | The complete installed radio and control system passes an inspector-supervised test |
| Weapon lock | Physically prevents dangerous actuation, extension, rotation, ignition, or movement outside the cage | A sturdy, visible lock with a warning tag can be inserted safely with one hand |
| Edge covers | Sharp points or edges capable of a serious cut or puncture must be covered during event handling | Transport covers are installed before the robot is moved around the venue and remain available for inspection |
1. A reachable master power cutoff
The master cutoff is the fastest way to make a robot safe when it is being carried, inspected, repaired, or placed in the arena. Under NHRL’s 2026 rules, it may be a switch or a removable link, but it must be reachable without taking the chassis apart, operable with one hand, and capable of deactivating the robot within 15 seconds. It must remove power from both the drive and weapon systems.
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For a 3 lb robot, NHRL permits battery connectors to serve as the removable link. A 12 lb or 30 lb robot needs a dedicated removable link with adequate strain relief. In those two classes, the cutoff must be accessible while the robot is in its primary fighting orientation. Design the opening, handle, and wiring into the chassis rather than treating the cutoff as an afterthought; an inspector should be able to find and operate it immediately.
A switch that leaves a weapon controller energized, or that allows lights and sounds to make an inactive robot appear active, creates confusion during handling. NHRL advises switching off all systems so the robot’s status is unambiguous. The rule describes performance and access, not a particular switch brand or connector, so select hardware that is mechanically secure and electrically rated for your system.
2. A tested radio failsafe
A failsafe is a response to lost control, not merely a feature printed on a receiver’s packaging. NHRL requires the entire installed control and communication system to pass a failsafe test overseen by a safety inspector. If the transmitter is powered down or the signal is lost, the drive must stop within 30 seconds and the weapon within 60 seconds under those rules.
Use a dedicated receiver for each robot and for each multibot segment. Configure the receiver, speed controllers, weapon controller, and any intermediate control board as one system, then test the actual transmitter, antennas, batteries, and wiring that will compete. A receiver advertised as having a failsafe does not automatically satisfy inspection: the event evaluates the behavior of the complete installation.
Even an autonomous robot needs a radio control system for remote activation and deactivation under NHRL’s rules. Autonomy therefore does not remove the requirement for a human-accessible shutdown path or an inspector-observed failsafe.
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3. A positive weapon lock for every dangerous mechanism
A weapon lock is a physical restraint for transport, pit work, inspection, and other times when the robot is outside the permitted match activity. It must prevent dangerous actuation, extension, rotation, ignition, or other movement. The requirement covers more than conventional spinning weapons: a CAM lifter or another mechanism that can move dangerously also needs a lock.
NHRL limits movement while a lock is fitted. A rotating kinetic weapon may move no more than 180 degrees. A non-kinetic weapon may move no more than 15 degrees, and any remaining movement must not create a pinch hazard. Meltybrain designs need wheel locks or covers. Projectile systems need a physical method that prevents a projectile from being expelled.
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Special rules for flame and heat systems
Flame systems require organizer approval of the lock system. NHRL describes possible controls including an inline valve, a switch, or a physical block, and requires covers over exposed hot-surface igniters or spark gaps. A fuel shutoff alone may not prevent ignition or residual movement, so the approved arrangement must address the complete hazard. Ask for approval before arriving with a flame design rather than assuming a lock that works for a spinner will be accepted.
4. Covers for dangerous edges
Edges and points that could easily cause a serious accidental cut or puncture must be covered while the robot is moved around the event. These covers are a handling and transport control, not a fighting attachment. They should be retained with the robot, installed before carrying it through the pits, and removable only when the organizer permits the fighting configuration.
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Power, batteries, and stored energy
Battery-voltage limit
NHRL’s 2026 rules limit nominal battery voltage to 75 volts unless the organizer grants explicit approval. A system that produces voltage above the applicable battery limit can require additional inspection even when the battery itself is within the limit. Calculate the nominal pack voltage from the cells and document any high-voltage circuits, capacitors, converters, or other energy-storage components for inspection.
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NHRL requires lithium-chemistry batteries to be charged with balance leads and says charging batteries must be attended. Do not leave a charging pack unattended in a pit or workshop. Use the event’s designated charging area and procedure, keep damaged packs out of service, and make the charger, leads, and connectors easy for staff to inspect.
Restricted and prohibited designs
Get approval before building around a restricted system
NHRL identifies pneumatic and hydraulic systems, certain exposed moving components above 300 mph, and designated combustion, flame, heat, projectile, inductive, or electromagnetic designs as restricted or subject to additional conditions. Builders are instructed to begin review at least four weeks before the competition so staff can review the design no less than three weeks before the event. Treat that schedule as a design milestone: submit drawings, pressures, materials, energy sources, and proposed locks early enough to make changes.
Written approval is safer than an informal conversation at check-in. If the organizer rejects a pressure vessel, exposed rotor, fuel system, or electromagnetic device, there may be no time to redesign after travel. A restricted feature can also change the required inspection, arena controls, or operating procedure.
Know what is prohibited
NHRL prohibits rocket motors, fireworks, radio jammers, and systems intended to harm people outside the cage. Its rules also address entanglement devices, arena-fouling materials, and other hazards. A legal weapon in one league is not automatically legal elsewhere, so compare the specific prohibited-items and arena-hazard sections rather than relying on past experience.
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Testing and pit safety are part of eligibility
Hardware controls are only one part of competition safety. NHRL states: “All robot weapon and drive testing must be done in a test box.” The test box requirement applies to powered testing of both the weapon and drive systems, subject to the narrow wheels-up exception described by the event. Do not spin a weapon or drive a robot loose on a pit table, floor, or parking area.
Power-tool work in the pits or machine shop requires safety glasses and other appropriate personal protective equipment. Use guards, secure the workpiece, and follow venue instructions for grinding, drilling, cutting, welding, and compressed gas. Keep the weapon lock installed while working on the robot unless the organizer has placed it in an approved test condition.
NHRL also states: “Robot control and communication systems must pass a fail-safe test overseen by a Safety Inspector at Safety Inspection to compete.” Plan to show the cutoff, failsafe, weapon lock, edge covers, batteries, and restricted-system paperwork in a calm, repeatable inspection sequence.
Inspection-day checklist
- Confirm the rule version. Record the event, weight class, and effective date of the rulebook. Check for amendments or a newer draft that has become effective.
- Demonstrate the cutoff. Show that one hand can reach and operate it without disassembly, that drive and weapon power are removed, and that the robot is inactive within the required time.
- Run the installed failsafe. With the robot secured in the approved test area, power down the transmitter or interrupt the signal and verify the drive and weapon stop within the event’s limits.
- Fit the weapon lock. Insert it without putting fingers in the danger path, attach the visible warning tag, and show that residual movement stays within the permitted limit.
- Install handling covers. Cover sharp edges and points before moving the robot through the venue.
- Present battery information. Know the nominal voltage, chemistry, balance-lead arrangement, and charging procedure. Keep charging attended.
- Bring approval records. Have written organizer approval for any pneumatic, hydraulic, high-speed exposed, combustion, flame, heat, projectile, inductive, or electromagnetic feature that requires it.
- Use the test box and PPE. Perform powered tests only where the organizer allows and wear the required eye and workshop protection.
Design differences by robot class and weapon
Weight class changes the implementation even when the safety principle is the same. In NHRL’s example, a 3 lb entry may use a battery connector as the removable link, while 12 lb and 30 lb entries need a dedicated, strain-relieved link accessible in the primary fighting orientation. A compact plastic-bodied 3 lb robot can therefore have a different cutoff layout from a heavier metal chassis.
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Weapon type changes the lock as well. A spinner needs a restraint that controls rotational travel; a lifter needs a block that prevents extension and pinch points; a meltybrain needs wheel protection; and a projectile system needs a physical barrier to ejection. Flame systems add fuel, ignition, and hot-surface controls. Class and weapon should be treated as separate design inputs in the safety review.
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How to compare two event rulebooks
When deciding whether an existing robot can enter another league, compare the same safety axes side by side:
- Cutoff type, access location, and required shutdown time.
- Failsafe trigger, drive-stop limit, weapon-stop limit, and whether an inspector must witness the test.
- Receiver, transmitter, and multibot communication requirements.
- Weapon-lock movement limits, tag wording or size, and rules for special weapons.
- Edge-cover requirements during transport and inspection.
- Battery-voltage ceiling, chemistry rules, balance leads, and attended charging.
- Restricted-system categories, approval documents, and submission deadlines.
- Test-box or test-area rules, wheels-up exceptions, and required PPE.
- Inspection process, effective rule version, and who has final authority at the venue.
Use the stricter requirement when preparing a robot for multiple events only if it does not conflict with the other organizer’s design. A safer mechanical lock or more accessible cutoff is generally portable; a timing, voltage, or weapon-category assumption is not.
Common ways otherwise sound robots fail inspection
The cutoff is hidden or only disconnects the battery
If an inspector cannot reach the cutoff without removing armor, or if a controller remains energized, redesign the access and switching path. Label the control and practice operating it in the robot’s fighting orientation.
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Check every signal path, including intermediate boards and separate weapon controllers. Test transmitter shutdown and actual signal loss with the robot secured, then repeat after changing batteries, antennas, or firmware. The installed configuration is what inspection evaluates.
The weapon lock is a warning tag without a physical restraint
A tag communicates status but does not stop motion. Add a rigid pin, block, cover, valve, or other approved restraint that limits travel and cannot be defeated by a motor command. Keep fingers outside the weapon path while fitting it.
Sharp edges are covered only inside the pit
Transport covers are needed while the robot is being moved around the event. Store them with the robot and install them before leaving the work area, not after an inspector points out an exposed point.
Restricted hardware is disclosed too late
Send the design for review at least four weeks before an NHRL event when a restricted feature is involved. Include enough technical detail for staff to assess stored energy, failure modes, and the proposed lock or containment system.
The practical answer
A competition-ready combat robot is designed to become harmless quickly, remain harmless when control is lost, and be physically restrained whenever people are near it. Build the master cutoff, failsafe, weapon lock, and edge covers into the chassis from the first layout. Then verify battery, testing, PPE, and approval requirements against the event’s current rulebook. For NHRL’s 2026 Open Rules, that means a one-hand, 15-second cutoff; a failsafe stopping drive within 30 seconds and weapons within 60 seconds; a visible positive weapon lock; covered hazardous edges; a nominal battery limit of 75 volts without approval; attended balanced charging; test-box operation; and early review of restricted systems.
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