48V relay question [RE-wrenches]

[Allan Sindelar]

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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