DC-13 Load Category
The category defined for switching electromagnets, solenoids and contactor coils fed with direct current. On control circuits built with 12/24V DC this is the real operating condition of the relay contact, and because the DC arc does not extinguish by itself it is one of the most demanding categories.
Alternating current has an advantage that makes the arc easier to extinguish: the current passes through zero every half cycle. In direct current there is no such zero crossing; when the contact opens the arc continues until the gap grows large enough. On an inductive DC load the energy stored in the coil feeds this arc and the contact surface erodes rapidly.
That is why the DC ratings of the same contact are markedly lower than its AC ratings, and they fall further as the voltage rises. A current that is acceptable at 24V DC is far riskier for the same contact at 48V DC.
- Mandatory measure: connect a freewheeling diode in parallel with DC coils; it suppresses the reverse voltage peak at the moment of interruption and shortens the arc.
- Sizing: if you are going to use a relay for which only an AC rating is given on a DC inductive load, reduce the current markedly and request the manufacturer's DC curve.
On a receiver supplied with 12/24V DC, if the whole control circuit is DC then this category is the main factor determining contact life; looking only at the ampere figure would be misleading.
The context in which this term is used: Wireless Crane Remote Control Selection Guide