Gil Price's 1979 MGB with 1986 5.0L Ford 302 Engine
1979 MGB with Ford 5.0L V8 (owner: Gil Price)

Opinion Column: Circuit Protection For Headlamps

as published in British V8 Newsletter, Volume XIV Issue 2, August 2006

by: Curtis Jacobson

Sometimes when you overhear advice you disagree with, the best thing to do is bite your tongue. What about when the advice is safety related? I hope I won't offend anyone too badly by butting into a conversation I overheard at this year's British V8 meet.

The advice concerned wiring and fuse protection for automotive headlamps. It went something like this:

"Whatever you do, don't put a fuse in the headlamp circuit! Having the headlights go out while driving
on a winding road at night because a wire bounced into contact with ground can be fatal. Having your
car burn up is only a financial loss, but having a fuse in the headlamp circuit could get you killed."

Okay, hold it right there! Let's dissect this advice.

Wired properly, no one fuse will turn off both headlamps.  To my knowledge, reputable modern car and truck manufacturers wire left and right headlamps on two different fuses (or, more generally and accurately: "circuit protection devices"). This wasn't always true, but it's true today. There are many feasible schemes for headlamp wiring to achieve this. For example, it's possible to wire headlamps with the high-beam/low-beam switch downstream from the circuit protection (e.g. so left low-beam and left high-beam share a fuse, etc.), but this approach is now obsolete or at least uncommon. On modern cars there are four discrete circuit protection devices for the four respective headlamp filaments. Redundancy enhances safety. Even if one lamp, wire or fuse fails, the driver can get out of the mountains with two headlights burning.

A short circuit "turns off the light" anyway.  If a headlamp lead "bounces into contact with ground" then that particular headlamp is going OFF until the short circuit is corrected regardless of whether there's a fuse or not.

The characteristic behavior of typical automotive fuses isn't so black and white as the warning above suggests. The scenario of hitting a pothole and having a fuse blow immediately frankly suggests that the fuse has probably been spec'd too conservatively. Believe it or not, that's not a common problem for real world cars and trucks. We don't think about automotive fuses as "slow blow fuses", but that's actually not too far off because fuses are thermal devices and there is always some lead-time before they blow. Sometimes the leadtime is surprisingly long because the tin element of the fuse needs both current and time to heat up to its melting temperature. As long as the fuse blows before damage is done elsewhere, this isn't a bad thing.

Digression: why would a car or truck builder select a fuse that's rated higher than its load's required amperage or that "blows slowly" in the context of a given circuit? Because momentary over-current situations aren't necessarily hazardous. OEM's don't want fuses to blow more often than necessary any more than you do. Furthermore, automotive wiring engineers are trained to select fuses to protect wiring, not loads. (The prevailing philosophy among OEM electrical engineers is that all "load" devices that "need protection" should have it already built-in. This extends to over-voltage, reverse polarity, and radio frequency interference protection too.) Since fuses at the main fuse block are there to protect the wiring, they can be spec'd based on wire conductor size, length, insulation material, etc., and not based on the actual intended loads. That said, OEM's typically bench-test the entire electrical system methodically by inducing or simulating pretty much every possible fault source to verify system integrity and safety. (I've always heard this called "The Cadillac Test".)

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It's the big exceptions that matter most.  Wires shouldn't "bounce into contact with ground" on a reasonably well-crafted car. I don't care how thick the insulation on your wire is, the most important criteria of craftsmanship in harness design and installation remains what the automotive industry calls "routing and clipping." Wires should be properly supported so they don't bounce into contact or even fret against anything that could cut or wear into them.

Do you need fuses if wires are properly routed? Yes, because accidents happen.

COLLISIONS HAPPEN. No part of any car is more likely to be damaged in a collision than the front corners - EXACTLY where the headlamps and headlamp wires are!   One could make a pretty good case that no wiring on your car needs circuit protection more than your headlamps because headlamp wires occupy an especially vulnerable location.

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I hesitate to include this point, but it's as reasonable as the warning that inspired this editorial: Just imagine you or your passenger is badly hurt and/or trapped in the car after an accident. A headlamp fuse could possibly prevent a rapidly-spreading post-accident electrical fire. In rare situations like that, fuses save lives and not just property.

TECHNICIAN-ERRORS HAPPEN. From experience, we know one of the most common causes of wire harness problems is mistakes during maintenance and repair: both crossed wires and also mechanical damage when heavier components are serviced. Headlamp wires are especially vulnerable due to their length, routing, and the frequency of headlamp replacement. Sometimes damaged or crossed headlamp wires aren't noticed until the lights are turned on. A friend who proofread this column for me reported that just one week previously he had replaced a gutted MGB dashboard because its owner crossed two headlamp wires.

A short-circuited wire at the far end of the fender could cause a fire MUCH closer at hand.  Let's first review the science. The amount of current in a DC circuit is limited by the total resistance of the entire circuit. A "short circuit" is one in which a "load" (such as a headlamp filament and it's downstream wiring) is removed. This causes total resistance of the (newly "shorted") circuit to go way-down and consequently the current through the circuit to go way-up. The current increases throughout ALL the remaining circuit components. Anywhere there's resistance, there will be a proportional amount of heat generation until something breaks down (or the circuit is otherwise broken.)

What's the weakest link? Sometimes the answer isn't intuitive! It could be a short little "jumper" lead (or for that matter a corroded and fatiqued little switch contact) or it could be a long section of wire. It could be localized near a termination or splice. Perhaps you accidentally nicked too many strands of cable with your wire strippers. On old cars, it could be a section of cable previously compromised by corrosion or mechanical fatique. Typically, the wires will get real hot. If insulation melts anywhere along a section of wire, it's likely an adjacent wire's insulation will melt too. For these reasons electrical fires spread rapidly and unpredictably, especially in old cars. Fuses are important safety equipment!   We count on fuses to be "the weakest link".

What about old British sports cars?  Left and right headlight power leads should be protected by separate fuses. If you've already installed a wiring kit that didn't include fuses for these leads, consider splicing in a secondary fuse block. (Four-fuse "accessory" blocks are readily available.) Thinking "schematically", fuses should be as close to the "supply" (or battery "positive" terminal) as possible. Remember that craftsmanship really matters! Take your time, measure twice and cut once, etc. If you can swap an engine you can certainly develop the skill and knowledge to re-wire your car, and this enhancement is worth doing. (Future articles in this newsletter will further cover wiring and circuit protection theory, materials, tools, and skills.) If you haven't already re-wired your car you may be wise to hold off and do the job carefully all at once, but the basic advice that headlamp power leads should have over-current protection still applies.

But wait!  You've been out to check your new BMW and you can't find a fuse labeled either "headlamps" or "front right low-beam". (You may not be able to find a fuse for any exterior light.) Why not? As long as I'm writing this diatribe I might as well point out newer circuit protection technologies are rapidly replacing automotive fuses - especially on lighting circuits.

If you're expecting me to discuss "automatic" circuit breakers here, sorry! (I don't like them.) Perhaps that topic will fit into a future article. Positive temperature coefficient (PTC) circuit protectors are more interesting, but they're not likely to change the way we do engine conversions.

What's the future?  Actually, many newer car and truck models already utilize solid-state circuit protection for the headlamps, and also for other exterior lighting. Fuses are becoming an obsolete technology, replaced by electronically controlled devices (specifically MOSFET chips) that function as relays within a Lighting Control Module ("LCM". German spellers calls it an "LKM".) Some car manufacturers build lighting circuit protection into their "body computers" or their "vehicle ECU's". Electronic circuit protection can be quite economical because it replaces relays, fuses, fuse blocks, and wiring between them.

Modern LCM's have other benefits. They generally reduce wire harness lead-count as part of a multiplexing scheme and they facilitate use of more cost-effective switches. They also facilitate adding nifty computer controlled features and diagnostic functions. One of my favorite examples is "lamp substitution". Suppose your right-hand high-beam filament burns out. Why shouldn't the LCM automatically and immediately "substitute" by turning on the right-hand low-beam (while also displaying a warning/diagnostic message on a dashboard display)? Now that's what I call "safety fast"!

Disclaimer: This page was researched and written by Curtis Jacobson. Views expressed are those of the author, and are provided without warrantee or guarantee. Apply at your own risk.  In case you're wondering, Curtis's personal experience includes designing wire harnesses and specifying circuit protection for Volvo heavy duty trucks (i.e. "eighteen wheelers"). After that assignment, he was particularly responsible for researching and specifying the LCM that is now used on all current North American and European Volvo Truck models. His MGB conversion project was completed first though, and it doesn't reflect all he subsequently learned.

copyright 2006 by Curtis Jacobson, Longmont CO. All rights reserved. This article may not be re-published in whole or in part by anyone except The British V8 Newsletter without explicit written consent of the author.

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