Technical Relays, Fuses and Circuit Breakers

We all know electrical safety is important, but all too often this is more of a 'do what I say, not what I do' mantra. So, y'all, who uses relays, fuses or circuit breakers out there?

If so, how many?

If not, why not?

 

Talkin' to the wrong guy here. I've been known to use duct tape, wire nuts and bread wrap wire ties

 

Sort of like flossing the teeth you want to keep. Just use relays, curcuit breakers and fuses on the circuits you don't want to burn up.

 

I have a breaker on the auxiliary power wire for my trailer plug, because it's a big wire that runs the whole length of the car directly from the battery, and I would really like it not to short out. With the breaker setup, I can switch it off when I don't need it, thus making it a dead wire most of the time.

 

Yes, small extra cost for extra protection. You would never forgive yourself if.................let's not go there.

 

Got them where they make a diffrence.

Fuses are completely useless until something goes wrong. You don't miss them until it does.

 

I use all of those items,better safe than sorry.

 

Wait. So you're implying people create circuits that don't have fuses? Are they nuts?

 

No, no, not nuts. Uneducated if they don't know how useful fuses are, idiots if they know but still don't use them.

 

Lots of stuff in our old cars weren’t fused from new…

I wonder how many understand how to select the correct fuse rating in a custom application?

The wrong size fuse is as useful as no fuse.

 

Taught automotive electrical for years, I use all the above.

 

I prefer cartridge fuses over breakers. I've had breakers weld themselves together to were a 15 amp breaker wouldn't open on a direct short. Good thing I had a pair of insolated Dykes to cut the circuit, that piece of 20 amp romax was glowing red like the inside of a space heater!

 

^^^^^ Romex!, in a car?

 

Remember using foil from chewing gum or cigarettes'..

LMAO!! No this was a house I was working in. It was the Kendall oil company owners mansion.. Actually Greenfield with stranded wire would work..

 

Pretty much all of the wiring kits come with the appropriate devices.........I use Ron Francis kits with very good results. Never had an issue in over 40 years of use.

 

A more pertinent question would be is how many actually understand how these items work and correctly apply them? Hint: it's fewer than you think....

No, just recognizing electrical reality. There's three circuits on virtually every vehicle that don't need to be fused (and weren't from the factory), and when they are it's almost always done wrong. People lose sight of the fact that the fuse isn't there to protect the connected device, it's there to protect the wire. A fuse won't do anything to prevent a failing device; at most it may limit damage to the device, but that's all. And for a fuse or circuit breaker to do it's job, it must be sized to the wire.

Those three circuits are ignition, charging system, and starting system.

The ignition system doesn't need fusing because if it fails, it will fail 'open'. It's really just a controlled dead short, with the coil being the weak link or 'fuse'. Or if you have electronic ignition, the module will fail to 'open', another 'fuse'.

Charging systems is where you see the most problems. Typically, most builders use #10 or #8 wire for the output from the generator/alternator to the battery, even with high output alternators. #10 is rated for 30 amps, #8 is 40. But to handle current spikes, say just after starting the vehicle, a much larger fuse will be needed. If you have a 100 amp alternator, you'll need a 125 amp fuse to prevent nuisance tripping but the fuse rating is at least three times the rated ampacity of the wire. It'll be a crapshoot as to which goes first; the wire up in smoke or the fuse. And again, charging systems are like ignition systems in that they also fail to 'open'. On a generator, any short or overload will kill the regulator, on an alternator it'll kill the regulator and/or the diodes.

Starting systems don't fail to 'open', but the problem here is the currents involved are much higher. The 'average' current draw on a 12V V8 starter is 250 amps, with spikes to 400 or more under certain conditions. Even 4/0 cable is only rated to about 200 amps, again you'll have a too-large fuse if sized to carry the load, not protect the wire.

The OEMs know this, and simply route any wire/cables to prevent mechanical damage and call it good. The only possible benefit with fusing these is in case of a catastrophic short circuit from an accident, and even then the fuse may not let go. You'll have other, more serious issues in any case...

 

It is true, the over current device is there to protect the circuit and not the device on the circuit. That is including the conductor. When looking at the current carrying capacity of copper conductors, the common though of 30 amps for a # 10 is with no more than three conductors in a cable or raceway. A single # 10 conductor in open air would be more like 50 amps. The single # 8 conductor in open air is more like 70 amps.

 

When it says 'open air', they mean literally outside in open air. Bundled wires and/or underhood or confined conditions with higher temps is another derate condition that should be applied. Also keep in mind that the wire terminations available are only rated to the lowest ampacity. Your electrical system is only as good as it's weakest link...

 

Well... partially agreed. Failures in the components of the systems will probably fail open most of the time. But you still have the wiring feeding it, and a wire with insulation worn through touching ground is a direct short to ground, not an open failure.

 

Yes, that's correct. But you don't have a solid connection, it's merely rubbing. Let me tell you a short tale...

Years ago I was working a remodel on a building (I'm a now-retired electrician). A carpenter with wild eyes runs up to me and says he 'shorted something' and shows me the sawzall he was using with a large crescent melted out of the center of the blade. I'm sure it made a big 'bang' when he did it. I tell him 'show me' and he takes me to where he was cutting some flooring out. Well, all there is to see is a slot in the floor with slightly blackened edges. It seems he cut into a conduit with a set of panel feeders in it, fed by 200 amp fuses. Not one of the fuses blew.... As the panel was scheduled to be removed, we didn't repair it and it worked just fine until disconnected. That sawzall blade was obviously rated for less than 200 amps...

I'd never seen any of these circuits fused until I came here. The OEMs aren't stupid, they built them unfused for years with no ill effects. They were generally trouble-free unless molested or if the harness 'aged out' and deteriorated to the point of bare wire showing. If you did have a bare wire that's rubbing, the most likely scenario is it would burn individual strands away until the wire breaks or possibly melt holes in the sheetmetal if a large-value fuse is used. A fuse that's rated for multiples of the wire ampacity will ignore this arcing and never blow. 'Everybody knows' (or should) that too large of a fuse can be an invitation for a fire yet install these too-large fuses in these circuits for 'safety'.... Sheesh!

 

Would I be incorrect to say a relay is used to compensate for an inadequate switch and also operate as a fuse in case of a short ?
I'm certainly not an electrician, but have wired many of my own cars over the last 60 years with heavy enough wire gage and switches and never had a problem. Maybe luck was better than skill? Always willing to learn.

 

A relay is nothing more than an electrically-operated switch. It has NO overcurrent protection. In fact, there's a danger that excess current can weld the contacts together and prevent the circuit from clearing even after turning the circuit 'off'. As to why you'd use one, there's multiple reasons, from having a switch not rated to switch the circuit current to operating multiple circuits with one switch to name just two. If you want to dip into the industrial relay pool, things like multi-pole and delay relays open up all sorts of possibilities, although you'll blanch at the costs of some of them. You also see more than a few relays used in aftermarket harnesses to compensate for inadequacies in the harness design unfortunately (dim headlights come to mind...).

And I'll point out here that NONE of the aftermarket plug-in relays are good for more than 30 amps continuous (regardless of manufacturer claims) because there is NO plug terminal rated beyond that. If you go over this number, use a rated relay with bolted load connections.

Lots more info here.... https://www.jalopyjournal.com/forum/threads/wiring-101.843579/

 

@Crazy Steve , this relay and socket have heavier connectors on the load side, than a 40A relay. Used in some Fords for AC, maybe some large trucks. Claim to hold 70A.


https://www.amazon.com/Cole-Hersee-RA-700112-DN-BX-Form_A-Bracket/dp/B01EJW5144

Any thoughts?

 

One can have a problem if you have a dump trailer, always a good idea to unplug the trailer from the pickup before dumping so it doesn't pull current from the pickup to feed the dump motor. I purposely do not have the charging wire hooked up on mine, as it will dump 3 or 4 times after putting a charger on the trailer battery

 

Just try to find terminals to fit that accept 70 amp wire....

I suspect these relays aren't designed for continuous duty, but have the higher rating to denote their ability to withstand high inrush currents from motor loads. You will have inrush current of 300% or more of running current when starting electric motors. This can substantially reduce contact life if the relay isn't rated for it. If the running current is 25 amps, this would be a good choice and #10 (30 amp) wire will be adequate.

Bosch makes a 30/40 amp relay that's rated 30 amp continuous, the 40 represents inrush current.

 

Here are the spec's:
https://m.littelfuse.com/products/d...in-relays/iso-power-relays/ra_700112_dn.aspx#

Looks like 70A/60A.

I have a source for sockets with proper terminals, also OE sockets as used in some A/C Ford products.

Thanks for responding, @Crazy Steve .

 

Where are you getting the terminals? All I could find when I looked was #10 wire size and they were unrated to boot.

 

Disclaimer: this is not my company I have nothing to do with these relays.

Saw this on bangshift today looks like these could clean up a lot of relays.
@Crazy Steve what do you think of them?

https://bangshift.com/bangshift1320...le-making-it-more-reliable-and-faster-acting/

 

Good thing is they won’t short out. The ones we used to use at a place I worked at had LEDs to indicate power on, we used them to activate 12v solenoids.
Mercy, just realized that was 35 years ago.
Thanks LB, I really didn’t need to feel so old just now...