What is a "relay box".

[No_RVSM_for_you]

For the electrical guru's...

Looking at the electrical chart, I see a DC Bus #1.

However, I also see a "Relay Box DC Bus 1".

Can you someone please explain to me at a 3 year old's level what a "relay box" does in this context and how a "Relay Box DC Bus 1" is different from just a "DC Bus 1".



Thanks,

[digits_]

Generally speaking a relay box is a box containing a bunch of relays. A relay allows you to control a high power dangerous circuit with a low power safe switch.

Very similar to a fuse box. You could think of it as a circuit breaker you can control remotely.

In your context, I would guess that the wires from your cockpit switches connect to the relay box on one side, and that the high power wires that control the actual systems (bus 1) would come out the other side

[No_RVSM_for_you]

Thanks, I've sorta got it

[Bede]

Generally speaking a relay box is a box containing a bunch of relays. A relay allows you to control a high power dangerous circuit with a low power safe switch.

Very similar to a fuse box. You could think of it as a circuit breaker you can control remotely.

In your context, I would guess that the wires from your cockpit switches connect to the relay box on one side, and that the high power wires that control the actual systems (bus 1) would come out the other side

Not entirely true. It doesn’t have to be dangerous- it’s just more current than you would ever want to run through a switch. It’s also not liKe a fuse box. A relay has no overcurrent protection- that is provided by a discrete circuit breaker.

A traditional relay is an electromagnetic switch. A cockpit switch or controller energizes a magnet that turns a switch on. Modern relays are solid state meaning that there’s no moving parts. They switch faster and have less electromagnetic interference.

[PilotDAR]

It's probably wise to focus on the function of the relay in the aircraft circuits, and the "box" part of the description as simply where to find it (either in the circuit, or in the plane).

As described, a relay will handle a current much greater than most cockpit switches. They also put higher current arching (worse at altitude) farther away from the pilot. Relays are also a failure point in the circuit, so you must understand that they are there, and could be the reason why something which should be energized - is not. For airplanes with more than one bus, relays are often used to tie them, again, because the whole bus (of circuits) carries more current than a switch could handle. They can also save cost and weight in running large, heavy, expensive copper wire a long distance, where a small wire can carry the signal to the relay, which switches the high current in a local area of the plane.

You may be able to help yourself understand this more by researching the symbols used for switches, relays, fuses and circuit breakers, then running your finger along the circuit diagram for the circuit you're talking about to see what you cross. Better yet, find a maintenance wiring diagram for the circuit, which shows the wire gauge. Bigger number = smaller wire. Very, very rough rule would be that switches would generally be used where the wire is ten gauge or higher number, relay if eight gauge or smaller - but there are certainly exceptions to that. But when you start to see two gauge wire, it's pretty certain there's a rely along that wire somewhere.

In terms of "box", yes, generally relays should be in boxes, unless they're tidy within an electrical panel itself - which is a box. Relays are very contamination sensitive so should be behind one or more layers of protection.

You may also see the word "contactor" which may also describe a relay. Think "starter contactor", which is a rely. In the case of GA planes, often bolted to the firewall, for car engines, right on the starter motor itself.

[co-joe]

Digits' explanation is decent, I'll go on to say that your coil voltage (or control) doesn't have to be a lower voltage than the one you are controlling.

Basically a relay is a switch, (in higher voltage/ current applications they are sometimes called "contactors"). They move a set of electrical contacts, On (closed) and off (open). Instead of you manually moving the switch position, you have what's called a coil, which causes electrical contacts to change state, either open or closed. The coil can be controlled by the same voltage as the power contacts, or it can be a totally different voltage. You can have an AC coil, causing contacts in a DC circuit to change state for example.

Newer relays can be "solid state", which means instead of the coil causing a plunger to move electrical contacts from one position to another, they use semi conductors, which are materials that act as an insulator (don't conduct electricity) unless enough voltage is applied to them, then they act as conductors. Because relays have moving parts, and because the contacts are subjected to repeated electrical arcs, they do wear out over time, so putting them all together in a relay box makes it easier to service them.

Hope that helps. TLDR...a relay is a switch

[digits_]

Not entirely true. It doesn’t have to be dangerous- it’s just more current than you would ever want to run through a switch.

Sure, it doesn't *have* to be dangerous, but isn't that often the main reason you wouldn't want more current to run through the switch?
Would you be happy having your master battery wires ran through your cockpit?

It’s also not liKe a fuse box. A relay has no overcurrent protection- that is provided by a discrete circuit breaker.

A traditional relay is an electromagnetic switch. A cockpit switch or controller energizes a magnet that turns a switch on. Modern relays are solid state meaning that there’s no moving parts. They switch faster and have less electromagnetic interference.

Agreed, but it kept the explanation simple, and the relay does offer protection. Maybe not against overcurrent situations, but it does protect the user against manipulating higher power circuits directly.

Since the original question was asking about relays on bus 1, chances are the higher power stuff on there could be quite nasty if a switch were to break and your finger started touching stuff or wires started to fall down on your head or lap.

The coil can be controlled by the same voltage as the power contacts, or it can be a totally different voltage. You can have an AC coil, causing contacts in a DC circuit to change state for example.

I agree, that's why I wrote power. But now I'm wondering... Even though it doesn't necessarily make much sense, have you encountered any examples where a high power circuit would close a low power circuit? Theoretically it should work but I can't imagine any practical applications.

[Bede]

I agree, that's why I wrote power.

Usually one would write "current" since current is the limiting factor for relays. Power is usually given for devices that dissipate heat.

But now I'm wondering... Even though it doesn't necessarily make much sense, have you encountered any examples where a high power circuit would close a low power circuit? Theoretically it should work but I can't imagine any practical applications.

Do you mean a higher voltage control signal controlling a lower switching voltage? Yes it's possible and rare. I took a look on digikey and there are a few available so obviously someone has a use for it in some weird setting. Maybe a 120 VAC PLC running some low voltage device.
https://www.te.com/commerce/DocumentDel ... ng=English

[digits_]

I agree, that's why I wrote power.

Usually one would write "current" since current is the limiting factor for relays. Power is usually given for devices that dissipate heat.

But now I'm wondering... Even though it doesn't necessarily make much sense, have you encountered any examples where a high power circuit would close a low power circuit? Theoretically it should work but I can't imagine any practical applications.

Do you mean a higher voltage control signal controlling a lower switching voltage? Yes it's possible and rare. I took a look on digikey and there are a few available so obviously someone has a use for it in some weird setting. Maybe a 120 VAC PLC running some low voltage device.
https://www.te.com/commerce/DocumentDel ... ng=English

I meant power as related to current * voltage.
I can understand the wisdom of using a high voltage relay to close a low voltage but high current circuit. Maybe I'm using the terminology wrong, but thinking about it, maybe a low power circuit could be used to close an extremely low power circuit that requires extreme precision, and needs some sort of specific sequence to close its circuit. Not that you would likely encounter that in an aircraft.

[PilotDAR]

A relay may be triggered by more than one source [of switching], so in being wired this way, do a job that a switch could not. One of the circuits I have designed is for a dual alternator (or generator) control system, where the failure of one alt/gen will automatically (by relay) turn off a non essential load, so as to reduce pilot workload, and preserve the remaining alt/gen for continued flight. I do this for high power consumption survey systems to comply with alternator fail design requirements. GPU circuits often include a relay, so that some airplane circuits are energized by plugging in ground power.

It's not terribly important that the pilot understand the voltages involved, as much as the currents, and thus the ammeter indications, and fuse/CB values. It is important that the pilot familiarize themselves with the electrical system as it is presented in the flight manual (usually section 7) and any flight manual supplements, which may describe an addition (and additional load and shedding) to the electrical system. Relays should be understood in the system as possible failure points. Other than that, there's not a great deal that a pilot can do about a relay failure, beyond what the abnormal procedures may state.

[Bede]

Digits,
Yes that's done lots (but not with a relay) when, if you could power a low power circuit directly using the output from a processor, but instead the engineer uses either a bipolar or field effect transistor to act as a switch to isolate the circuits to some extent. Relays are generally big, slow, and prone to failure (although solid state relays are better - but have other limitations). If the engineer wants true isolation, they use an optoisolator which is an led and phototransistor inside of a case. No electrical current passes between these two sides- the signal is transmitted optically. This device prevents any signal from going back to the control side.

[digits_]

Cool thanks!

[PilotDAR]

I can specify an aircraft qualified relay in a design, and approve it. I can specify other semi conductors and advanced electronic devices, and be asked to demonstrate DO 160 environmental and reliability qualification.... and maybe approve it. I believe that today's advanced electronic components are magnificent design solutions, but showing compliance to very old design requirements with them can be surprisingly complex!

[plhought]

For what it’s worth there’s plenty of cockpit switches in King Airs/1900s with 10-15 (in some older SNs - 20!) amps @28VDC going through them.

The switches are the CBs!

Considering lots of GA planes ripping around with only 60 amp alternators…

For what it’s worth - I usually associate relays with "control" vs “contactors” as the using little amperage to connect big amperage.