DC Isolators

Hi All,

I am a little confused around Current ratings on DC Isolators.

Last week I watched a you tube video from on the supposed differences between AC & DC Isolators.

The demo was with Scame 590 series

A 20A AC isolator and an identical one marked 16A for DC. Identical contacts, materials etc.

The DC uint was only said to be 16A rated with 2 contacts in series.

I have never really thought of contacts in series as a means of increasing current capacity, more parallel contacts/cables etc for this application.

The suggestion in the video was not that current was increased but the series contacts provided more air gaps in the circuit, thus it was better able to quench the Arc.

I am tempted to do a little test see the effects on the first and subsequent contact at FLC.

I have worked in and with DC for most of my career in one way or another but never really considered Arc damage.   

Can anyone point me toward some calculations for determining the DC rating of contacts from a given AC Rating for a similar utilisation code AC22/DC22 for example?

Here is an example of an Isolator claiming 63A rating but at the same time 32A on a per contact basis. 




  • Further to the comments of others above

    A small AC contact breaking a modest load, is essentially current rated by heating effects (contact resistance when closed) and providing a gap is established with a half cycle or so that is more than a new arc can be established, then the arc goes out at the first zero crossing of the load current.

    So the area of the metal in the on state sets the current, and the contact gap when open sets the highest voltage, and the speed of the mechanics  and the gas type and pressure sets the maximum AC frequency. (I do a lot of RF switching, which certainly is an AC, but arcs do not extinguish in anything like the same way. In that sense 50/60Hz is simpler)

    Realise that there is no such periodic extinction and re-ignition of the plasma when breaking DC so  the volume of plasma (== size of the arc) that can be produced relates to the total power available to generate the plasma - not just the current or the voltage, but rather the product. Actually anyone who has played with a simple buzz-box  arc welder will have seen this, but maybe not realised it - the 'thick plate' settings of ~ 200 hundred amps throws a far longer arc, and has a much higher current and more power consumption , but essentially the same voltage (perhaps 50-70 open circuit and 20-40 on weld) as the 50A/1.6mm setting.

    Contacts in series require the source to maintain a total arc volume that is 'N' times bigger - so you may increase the current or the voltage so long as you do not approach either the current or voltage limits set in the AC case. In general breaking DC with moving contacts is hard.

    As this video from Roobert 33 illustrates the issue rather well, breaking the same load current (4 mains heater elements) supplied first with AC, then with DC at the same voltage as the RMS of the AC - on DC the arc is really quite noticeable. Hence the glove....

    He could have used a magnet to push the arc sideways onto some sort of toothed plasma cutting surface, and DC switches often do, he does not, to illustrate the point.


  • And hence why we got into the habit of having switched socket outlets - when many early supplies were DC.

       - Andy.

  • And hence why we got into the habit of having switched socket outlets - when many early supplies were DC.

       - Andy.

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