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DC Isolators

Martyn

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. 

  

Thanks

Martyn.

Parents

  • Multiple contacts in series will not increase the steady state current carrying capacity. Multiple series contacts WILL help tp increase the DC current that can be switched because the arc is spread over 2 or more contact gaps and thus more easily broken.

    An old but still useful rule of thumb is that contacts in switches and circuit breakers that are rated for AC, may be used on DC if the voltage is reduced to about 10% of the AC rating.

    For example when wiring an off grid home for 24 volts nominal DC, I would use standard types of "240 volt AC " light switch.. On 50/52 volts I MIGHT tolerate 240 volt ac switches but only under favourable conditions such as a non inductive load, and an operating current much less than the nominal rating. On 110/120 volt DC no way!

Reply

  • Multiple contacts in series will not increase the steady state current carrying capacity. Multiple series contacts WILL help tp increase the DC current that can be switched because the arc is spread over 2 or more contact gaps and thus more easily broken.

    An old but still useful rule of thumb is that contacts in switches and circuit breakers that are rated for AC, may be used on DC if the voltage is reduced to about 10% of the AC rating.

    For example when wiring an off grid home for 24 volts nominal DC, I would use standard types of "240 volt AC " light switch.. On 50/52 volts I MIGHT tolerate 240 volt ac switches but only under favourable conditions such as a non inductive load, and an operating current much less than the nominal rating. On 110/120 volt DC no way!

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