What Do Amp Ratings on Automotive Relays Mean?

[Alfa romeo junior]

Hi,

Could someone explain to me what amps mean on the automotive relay? Here is the picture of my 30 and 20-amp relays.

 

[nsaspook]

Hi,

Could someone explain to me what amps mean on the automotive relay? Here is the picture of my 30 and 20-amp relays.
View attachment 359833

It's rating for what current the relay contacts can handle safely and continuously. If you had a car headlight that needed 25A in normal operation, you would pick the 30A relay to control it, not the 20A.

 

[Alfa romeo junior]

Thanks, nsaspook for the explanation. Is that current between contacts 85 and 86, or between 87 and 30?

 

[nsaspook]

Yes for 87 and 30. If you use a relay like this, the wiring for those contacts needs a fuse (at the electrical power source or battery) selected to protect the wiring from overloading or short circuits.

 

[Alfa romeo junior]

Thanksss. Now I understood 100%.

 

[Baluncore]

Welcome to PF.

The challenge with switching DC current, is with turning the current off, then the arc that forms in the air between the open contacts, must be extinguishing. The arc forms due to inductance in the circuit.

DC currents are about ten times harder to switch than AC, because with AC, the arc is extinguished at the end of each AC half-cycle, as the current changes flow direction.

To reduce arc burning in air, relay contacts must contain expensive heavy metals like platinum, gold, and silver. Before the RoHS in 2006, relay contacts often included cadmium, which is a toxic metal that contaminate old relays.
https://en.wikipedia.org/wiki/RoHS

Currents in automotive circuits are now switched with Solid State Relays, a semiconductor MOSFET switch, rather than by conductive metal switches with an air gap.

 

[Lnewqban]

Thanksss. Now I understood 100%.

Welcome!

Also, see:
https://www.12voltplanet.co.uk/rela...a7VFwCBJGYKDnMsV4AMokwugNZz5w7SwK-r2LMh1JCmzS

 

[Alfa romeo junior]

Thanks all. I am happy that I have registed here.

 

[nsaspook]

Remember these types of relays were designed to be used in cars with 12VDC electrical systems.
Outside of this applications YMMV.

@12 VDC, with resistive loads (with inductive loads there can be a HV back EMF kickback across the contacts that can arc), the arc potential during load switching is minimal unless the breaking currents are really high (thousand+ of A). There just is not the field potential across the arc to sustain the conduction plasma until you get about ~30 VDC. You will get contact metal resistive heating and damage to the conductors when touching at high currents but little or no sustained plasma channel arcing when they separate. Just a lot of sparks from vaporizing metal.

https://brainfiller.com/arcflashforum/viewtopic.php?f=4&t=4867

Hertha Ayrton in The Electric Arc has a formula that gives an absolute minimum voltage of about 28 VDC. This is a very old book and tough going but the formula is in there. Above that point you need a minimum current as well so it becomes dependent on fault current but reasonable short circuit currents such as 200% of the power supply maximum output quickly show why there are practical limits. Ammerman covers this and more in detail here: DC Arc Models and Incident Energy Calculations. R. Ammerman, T. Gammon, P.K. Sen, J. Nelson. IEEE Transactions on Industry Applications, Vol. 46, No. 5, September/October 2010

Essentially the minimum theoretical arc voltage is around 25-30 VDC but this is with nearly infinite available current and working at a 1 mm gap. One of the more recent papers (referenced above) is the Stokes and Oppelander (1991) work that reviewed a lot of the others and gives a formula for arc voltage as Varc=(20+0.534*L)*I^0.12 where L is the arc gap in millimeters. So if we set the gap to 1 mm then to get to a 50 V arcing fault we'd need 2.435 = I^0.12. Taking the log of both sides we get 0.3865 = 0.12 * log(I) or 3.221 = log(I). Solving finally for I we get 1662 A which is quite a bit more current than most DC power supplies can put out. Don't forget that this is with a ZERO system resistance. The real system would need at least twice that current based on maximum power transfer arguments so we need at least 3324 A to sustain an arc at 50 VDC with no gap at all across air. Welders do not need this much current because the arc is within a metal vapor/liquid. Note that pretty much all the models in the above paper show an absolute theoretical minimum of around 25-30 VDC for arcing no matter how much current is applied even as the arc gap drops. You can run through Ammerman's full theoretical model working it to see where the cutoff is but it's there and there's no way to worry about anything under around 50 V. So the limit in 70E stands.

 

[Baluncore]

Just a lot of sparks from vaporizing metal.

That metal vapour condenses on nearby insulation, reducing the spark gap and the life of the relay.

Polymers in the vicinity of the arc, are pyrolysed by the arc UV and heat, to carbon, that then supports a low voltage arc.

Arc analysis, studies stabilised arcs, with fixed gaps, in resistive circuits. Real arcs are ignited by the opening of a gap from zero separation, in a circuit with significant inductance. A transient arc, can accumulate damage sufficiently, to become supportive of a stable arc.

 

[dharm34]

Looking at your relays, the "amps" rating, like the "30A" and "20A" you see printed on them, indicates the maximum electrical current that the relay's contacts can safely handle when they are closed. For the 30A relay, it can switch circuits carrying up to 30 amperes of current without damaging itself, while the 20A relay is rated for a maximum of 20 amperes. Exceeding this rating can cause the relay's internal contacts to overheat, melt, or even weld shut, leading to failure of the circuit it's controlling.