48V relay question [RE-wrenches]

[Windy Dankoff, Dankoff Solar]

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Allan,

I recommended two single-pole relays on the assumption that you would 
want a double-pole relay. The two relays together would produce the 
equivalent of one double-pole relay. I do think it's overkill to run 
through FOUR series contacts, and also, the more poles, the stronger 
the coil has to be, and the more power it uses.

A lot of single-pole relays actually have a 2-series contact 
structure, like pressure switches do. You can look in Grainger and 
see what I mean.

Windy

At 9:15 PM -0600 10/19/03, Allan Sindelar wrote:
>This idea makes sense--sort of. I can work with two 24V relay coils wired in
>series on 48V. But why do you recommend two 24V single-pole relays, then
>discuss wiring the relay contact circuit in series using two-pole relays? It
>would seem to me that double-pole relays (which are just as readily
>available via Grainger and other sources) would allow a total of FOUR series
>contacts when wired in series for 48V. Am I missing something here? Is it
>just overkill and not needed at 48V?
>
>Thanks,
>Allan at Pos E
>
>----- Original Message -----
>From: "Windy Dankoff, Dankoff Solar" <windy at dankoffsolar.com>
>
>>  1. Install a double-throw relay(s) to act as a transfer switch for
>>  the PV circuit. It switches the array to either the pump controller,
>>  or to the battery system's array input.
>>
>>  2. Connect the relay coil to battery power through the float switch
>>  in the water tank. The float switch can be either NO or NC, just
>>  configure the relay contacts accordingly.
>>
>>  When the tank fills, relay is actuated, and transfers the PV charge
>>  to battery system.
>>
>>  48V relay -- yes, hard to find. Alternative is to get two single-pole
>>  24V relays, and wire their coils in series. This is the exact
>>  equivalent (given same contact rating) of a 48V 2-pole relay.
>>
>>
>>  About 2-pole transfer relays:
>>
>>  Two poles are generally used when you break both + and - (or both hot
>>  sides of AC). However, if the system has negative ground (like AC
>>  neutral), you are NOT supposed to break the grounded conductor (NEC).
>>
>>  Breaking both conductors, people often say, helps to reduce the
>>  arcing at the switch contacts, so what do you do? -- OK, it's true,
>>  BUT switching through two sets of contacts reduces the arcing no
>>  matter WHERE they are located in the circuit. So, wire the relay
>>  contact in series, BOTH on the positive line. This helps break the
>>  arc when the contacts break. (It distributes the voltage differential
>>  between the multiple contacts.)
>>
>>  You don't have to go way overboard on relay contact size or quality,
>>  because it is unlikely to switch more than about once per day.
>>
>>  Windy
>
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