When comparing switching devices for AC and DC use you can specify the acceptable maximum voltages for a common current eg
or the acceptable maximum current for a common voltage eg
The latter method is usually MUCH less useful.
ie almost nobody cares about 230 VDC and few would care about eg 24 VAC.
Specifying current is usually more useful.
eg for a given system of voltage you usually know if you want to swutch 1A ir 10A or 50A etc.
Re
Does this current rating apply to both the rated AC and DC voltages?
For a reputable specification the spec applies in a logical manner as stated.
So if it says 10A, 230 VAC, 32 VDC you can expect the 10A to apply to both.
Occasionally a manufacturer MAY indeed spec both different Voltage and current eg
12 VDC, 10A or 230 VAC, 20A
usually because the voltages concerned are standard ones which buyers are liable to be interested in. eg here "car battery" type voltages and mains type voltages.
re
What is the highest current I could reasonably expect from a DC switch before moving to a relay?
Whatever the manufacturer says.
You can buy switches of almost any rating.
10A is common.
50A is more specialised but not wholly unknown.
500A would be very very very specialised and you probably want to be wearing a specialised suit when using it (really) BUT they exist.
Realays are not special as of right in switching ability.
They exist more because of their remote controllability than because of any ability to switch better.
Switches that are liable to draw an arc that prevents switch "opening" will also have mechanisms to stop this happening.
This may be as simple as spring loaded flick action which many people will have encountered without realising why they act as they do, through to units with magnetic blowout and arc paths which cause the arc to climb, lengthen and self extinguish. The latter are more the realm of very high capacity breakers but could be used i switches for the brave and foolish if desired.
FWIW there are contactors which may also be used in "switch" mode that are so hazardous that they will usually kill a person in the vicinity if they break rated fault current while people are in the vicinity and which, if they need to be manually operated, require special suits, long poles and lots of luck and still may kill people ... !
Glossary: Z ~~= s when the world is upside down.
Dry contact means no energy is supplied to the contacts (completely passive and isolated). In order for the DAQ to read it, it needs to have a defined voltage, Either a pull up to the positive supply (logic 1) or pull down resistors to the negative supply (logic 0) or an additional IO. Usually, DAQs have a reference voltage output, you can use that one, or alternately, an output line, set to a known logic level. Supply voltage is relative to the DAQ supply, either its ground or Vcc.
As long as the cable between the alarm and IO board is relatively short, you should not have any problems.
Best Answer
I would imagine that the current derating at higher voltages would be due to arcing when the contacts open. Keeping the same current as voltages increase will allow arcs to persist longer and cause more damage to the contact surfaces. On small relays and contactors, these arcs are tiny, but if viewed in a darkened room, you can see that they do exist. Over several thousand cycles, (especially with inductive loads such as motors) the arcing will cause pitting and oxidation of the contact surfaces. Damaged surfaces are more resistive, which heats the contacts, and promotes more arcs. Failure will come much sooner under these conditions of accelerated wear.