Martyn said:

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.

Yes, that is the answer ... it's to do with the fact that with 50 Hz AC currents, the arc current passes through zero 100 times per second, and it is therefore more difficult  to sustain an AC arc. With DC, for on-load isolation, it's really difficult to break any current, as an arc is far more simple to draw and sustain over relatively long distances.

Martyn said:

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?

I don't think there is one, unless there are academic papers that discuss the issue? Certainly nothing I've come across recently.

I understand the request is because you're interested, but the reason I don't think it would be readily available is that, according to the product standards and product selection the manufacturer must apply the rating for AC and/or DC, and if the product only has ratings for AC, it must not be used for DC (i.e. there's no need to know).

I do, however, have a suggestion. If you gather a number of data sheets for different products that have AC and DC ratings for on-load isolation ("load break operation") and functional switching , you could pop the information in a spreadsheet and see if there's a correlation (even if only a rough general relationship). You could choose isolators, switches, relays etc. ... and even look at correlating "one pole only" vs "two poles in series" recommendations.

Martyn said:

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.

Yes, that is the answer ... it's to do with the fact that with 50 Hz AC currents, the arc current passes through zero 100 times per second, and it is therefore more difficult  to sustain an AC arc. With DC, for on-load isolation, it's really difficult to break any current, as an arc is far more simple to draw and sustain over relatively long distances.

Martyn said:

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?

I don't think there is one, unless there are academic papers that discuss the issue? Certainly nothing I've come across recently.

I understand the request is because you're interested, but the reason I don't think it would be readily available is that, according to the product standards and product selection the manufacturer must apply the rating for AC and/or DC, and if the product only has ratings for AC, it must not be used for DC (i.e. there's no need to know).

I do, however, have a suggestion. If you gather a number of data sheets for different products that have AC and DC ratings for on-load isolation ("load break operation") and functional switching , you could pop the information in a spreadsheet and see if there's a correlation (even if only a rough general relationship). You could choose isolators, switches, relays etc. ... and even look at correlating "one pole only" vs "two poles in series" recommendations.