Siberia Racing
Tech Pages

OVERCURRENT Protection- Breakers and Fuses

This article is an updated version of the one I wrote several years ago for "Scale Auto Journal." It illustrates the need for a proper overcurrent protection system on any slot car track. Even if you only use "wall warts" for power, overcurrent protection is still important to protect your investment.

Overcurrent protection is essential on any track especially if equipped with battery power. Unlike power supplies, batteries are stored energy devices and contain a significant amount of energy even when not in use. A battery in a poor state of charge can deliver enough energy to cause damage to even the best track wiring if proper overcurrent protection is not provided. A track's overcurrent protection system must protect the cars and controllers as well as the track wiring. This article will discuss the proper sizing and location of fuses and/or circuit breakers.

Each lane should be equipped with overcurrent protection to minimize any damage caused by a short or fault. I've personally seen the result of a track meltdown caused by a miswired controller. The track wiring was first rate, however, no lane or battery protection was installed. During a race, a controller failed causing a dead short across the battery. In shock the driver dropped the controller and did not unplug or isolate the fault.  An overcurrent device would have opened causing an interruption in the race but saving the controller and the track. In this case, the controller melted and the track's main power switch exploded!  The switch failure saved the track but destroyed itself in the process.  Fortunately, no one was hurt when the switch detonated.  

In this best case scenario, the track wiring was damaged, the race was over, and many hours of work went up in smoke! In an unattended worst case scenario, with fresh batteries, the battery can supply enough current to start the cable insulation on fire in the event of an unprotected short. An unattended cable fire may eventually start the track frame or, if you're like me, the stuff stored under the track in cardboard boxes on fire with predictable results.  For a more spectacular event see the video of what a short circuit can do to a poorly protected system if enough current is available.  Its a 5MB file and may take some time to load on a dialup but its worth the look if you can afford the time.  Its a bit overkill but hopefully it illustrates the point that proper overcurrent protection is a really good idea.

Even the pros can hook up backwards with disastrous results. In practice at a HOPRA National race several years ago multiple main power relays failed under the Restricted Open. track when, once again, somebody repeatedly hooked up backwards in practice. You think that somebody running RO would know how to hook up a controller.  Nope!  The scenario went like this. The driver walked up and occasionally hooked up backwards faulting the track when he hits the brake. The relay eventually fails, the track dies and the driver unhooks the controller and walks away apparently not knowing that he caused the problem. A new relay is procured, replaced and the cycle repeats. They had about four of these things fail and didn't know what to do. 

The relay was rated at 20A and the track had a 15A main breaker. However, every time, the relay protected the breaker by failing first. This is not a surprise as relays and breakers are not rated the same and relays are not designed to interrupt faults without damage. The problem was "solved" by modifying the relay and replacing the melted 18 AWG jumper wires on the 20A relay with 14 AWG ultra flexible RC car lead wire. I don't know if that "solved" the problem but I know that the relay never failed again and the next time our driver hooked up backwards he held his own mini Chernobyl in his hand as his controller melted down. The 15A breaker never opened.  There was nothing wrong with the breaker. Read on to find out why it never opened when the controller bought the farm.

A "successful" solution but not optimum as had the fault been on the track instead of at the drivers station, the 16-18 AWG power jumpers to the track rails would have most probably opened ruining the track and ending the race right then and there.  At a recent HOPRA nats a front axle came apart and the axle ended up across the rails.  The track was protected by a similar15A breaker but again the breaker didn't open and the track was damaged by the heat generated by the current going through the rails and the axle. Get the picture that just having a breaker doesn't guarantee protection.  You have to have the right size and type of device.

I recently learned that even if you use an electronic power supply with built in electronic overcurrent protection you still need fuses or breakers as the power supplies protective circuitry may not save itself from damage during a sustained overload. At a recent race I had a 4.5 Amp and a 10 Amp power supply hooked in parallel providing 14.5 Amps to the track.  At some point during the day somebody hooked up backwards and repeatedly faulted the track during a race.  Supposedly there was a main fuse but the power supplies could not provide the current to open it and the power supplies folded back multiple times until the power was turned off.  When an electronic power supply folds back it reduces the output voltage to maintain the output current at its preset maximum. Well the idea sounded good, however, when I got home I found that the 4.5 Amp supply had failed due to the multiple overloads. The unit was repairable but the repairs cost $25.00 along with over a month of down time.  A main power fuse isn't currently installed at CRR as I have faith in my wiring and in my electronic power supplies electronic overcurrent protection. However, because of this recent experience, I will be installing a main 30 Amp fast-acting fuse immediately downstream of the positive power supply terminal prior to the next race at CRR.  

Some of you will say that a fuse will decrease the voltage to the track and your car won't go as fast. A fuse is a resistive element and will convert a small amount of the current going through it into heat. The voltage drop across a 5 Amp fuse carrying a 2 Amp load is 0.019 Volts. Assuming that a car requires 5 Amps, the use of a single 5 Amp fuse per lane would lower the voltage at the lane by under a tenth of a volt. A small price to pay for protection! And, since fuses are provided for all lanes, it's the same for everybody.  

How is a 5 Amp fuse sufficient when some rules require the track's power supply to be able to continuously supply 6+ Amps per lane at 18 Volts?  I don't know how the rule was developed, however, I know that, if properly tested in accordance with the rule, no battery system in the world without an 18V power supply in parallel will pass as the output of a lead-calcium or lead-acid battery upon discharge cannot exceed its no-load or open circuit voltage.  The open circuit voltage of a lead-acid battery is 18V.  The open circuit voltage of a lead-calcium pack is slightly higher.  Battery voltage will decrease when load is applied and will continue to decrease as the battery is depleted.

I have also yet to see an H.O. car that requires 5 Amps continuously.  I have seen some cars that require 5 Amps accelerating from a stop or coming out of a slow corner.  I have found that even unlimited cars work fine and don't cause false trips or fuse failures when protected with a 5 Amp fuse.

A 20-30 Amp slow-blow/time delay main battery fuse mounted on the jumper between the six volt and the twelve volt batteries in conjunction with a 5 Amp fast-acting fuse (or magnetic only breaker) per lane will protect your track and still supply the most power hungry car with all the power it needs. Mounting the main battery fuse on the jumper between the batteries allows the fuse to protect the entire electrical system including the battery. Because of its time/current characteristics a 15 Amp breaker is not recommended for main battery protection. Selected properly, circuit breakers, do work well for individual lane protection. If you choose breakers, don't use the automatic reset type. Use a breaker that can be manually reset.

It's essential that the overcurrent protection system be sized to protect all of the track wiring. A 20 Amp fuse will adequately protect 14 AWG wiring. However, the 20 Amp fuse will not protect the small gauge 18-20 AWG jumpers from driver's stations to the track. In the event of a short, the jumper will be damaged or may burn open before the 20 Amp fuse opens. That's why individual lane protection is important. The 5 Amp fuse is designed to adequately protect the smaller gage jumper wires associated with each lane. The larger main fuse is there only to protect the batteries and the large gauge wiring from the batteries up to the 5 Amp fuses. The main battery fuse by itself will not adequately provide full protection.

Fuses and breakers are rated by the maximum continuous current that they can carry. Their minimum tripping or opening currents are typically 125%, 150% or even 200% higher than their continuous rating,  The device may also have a designed in time delay to preclude false tripping in the event of a short overcurrent event.. Figure 1 is taken from BUSSMAN fuse bulletin SFB and indicates the time current characteristics of the Buss AGC line of small glass  fuses.  Unfortunately, its a big graphic and takes time to load.  It can be seen from the figure that a 5 Amp fuse will actually take 140% of its rating or 7 Amps for 300 Seconds (five minutes) without opening. These ratings are typical for both fuses and breakers. A 5 A mp fuse needs a fault of over 20 Amps to open instantaneously (less than 0.10 seconds) and a 15 Amp breaker needs a fault in excess of 150 Amps to open in that time!  That is why the 15 Amp breaker never opened in the above scenario and why I recommend no larger than a 5 Amp fuse or breaker to protect each lane. Make sure your breaker or fuse is designed for DC service.   The 15A breakers in the above scenarios were designed for 120V AC service.  The breaker's internal thermal element was designed with that voltage and type of service in mind.  When used on 20V DC that 15A breaker would act more like a 50A breaker than a 15A breaker.  Another reason why those 15A breakers didn't work when called upon.


Figure 1
Buss AGC Fuse Time Current Curves


If your track just runs stock motors you might consider this. Wire the filaments of a single #1157 12 Volt automotive bulb in parallel instead of a fuse for each lane. The bulb allows just enough power for a stock motor but, a rewind, or dewind or a bound-up motor (requiring too much power) will draw too much current causing the light to glow alerting the driver and slowing the car down. Like fuses, the bulb doesn't require a significant amount of voltage until the filament heats to incandescence. I wired an oval using lights instead of fuses and never blew up a stock motor by running it bound up or too low. In the event of an overload the light bulb glows and limits the current thus saving the motor. When racing, the light bulb acts like a "Restrictor Plate'' equalizing the competition. This idea came from overseas where some 1/24th clubs use a similar idea for their stock or restricted power classes. A main battery fuse is still required as the light bulbs won't protect the wiring between the bulbs and the battery.

If you want indication when a fuse blows (or a breaker opens), wire a #57 12 Volt automotive bulb across the fuse (breaker) terminals. When wired like this the light bulb will act as a "blown fuse" indicator and will glow if the fuse fails and the car is still on the track. A better system is to install the blown fuse alarm as described elsewhere in the Tech-Pages. A blown fuse alarm or indicator will allow the driver to alert the race director and save the racer from unnecessarily pulling the car or controller apart in the event of a blown fuse.

Fuses or breakers will protect your track if properly selected and installed. Fuses and fuse holders are available in many sizes at electronics or auto parts stores.  Figure 2 shows the ideal setup (main fuse located between the batteries with individual fuses (or circuit breakers).  Fuses do need to be replaced when they blow. Breakers don't need replacement but are harder to find in the smaller sizes required for HO. Siberia International Raceway used 3A breakers at each lane and never had a breaker spuriously trip during a race. CRR's power panel uses Heinemann aircraft type 5 Amp breakers to protect each lane. These breakers are designed to carry 5 Amps continuously but to trip instantaneously at 6.25 Amps. During CRR's first race the breakers tripped several times in practice when a Pro T-jet with ski-shoe pickups spun shorting the lane and during the semis when a controller failed. None of these were spurious trips and the breakers protected the track and did the job they were designed for.  To immediately alert the driver and race director when a breaker trips the breakers have been equipped with a self resetting audible breaker trip alarm.


Figure 2
Main Power Single Line Diagram


Protected or unprotected, fuses or breakers, power indicating lights or tripped breaker alarms, the choice is yours. I hopefully this article has shed some light on the subject allowing you to make an educated decision.

Steve "Maddman" Medanic

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Revised 7/10/2006