Solid state relay questions

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  My water comes from a well. When the well was drilled I had only
built my shop and so the well is now powered by my shop. The pressure
tank and switch is also at the shop.
   Now that the house is built I need to power the well from the
house. This is fine but I am not gonna move the pressure tank and the
pump switch must be at the tank for proper operation.
   The problem I must address is how to use the switch at the tank
while using power from the house. I cannot run power from the house to
the tank and then to the well pump because the voltage drop would be
too great due to the much longer run of wire.
   My plan is to instead use the existing wires coming from the shop
and going to the well to just carry switching power, not pump power. I
want to use a couple solid state relays, one for each leg of the 240
power, to switch the power to the well. There is plenty of room to
mount two solid state relays inside the well junction box. And I found
AC controlled solid state relays that use 80 to 280 volts AC  for
control and will switch up to 480 volts AC at 25 amps. My well pump is
a 2 HP pump that draws less than 10 amps.  It is a capacitor start
type pump so it draws less current at start up than a non-capacitor
type pump.
   So, my questions: Are solid state relays suitable for this type of
work? Are they typically able to handle surge current loads from motor
starting? Here is a link to the relay in question:

Re: Solid state relay questions says...
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yes it will handle it fine..

Actually in most systems these days they use Thyristor base switch for  
the starting cap where the gate gets its control signal from a reed  
switch that has a couple of turns of wire wrapped around it. The idea is  
the current of the one of the main leds goes thruogh this and when the  
current is high due to initial starting the reed switch closes which  
then turns on the Triac that connects the startnig cap to the third leg.

 So it works well there and running the two main phases should work fine  
also via a triac which is basically what you are talking about, just in  
a nicer package ..

Re: Solid state relay questions
jamie says...
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I wanted to add ..

 If for some reason these Relays show on, your pump will not shut off..
so this is something to think about..

Re: Solid state relay questions
On Sun, 9 Aug 2020 18:41:33 -0400, M Philbrook

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I would second that.

SSR's seem prone to fail shorted and that could be bad news.  A couple
of electromechanical sealed 20 amp relays make more sense IMO.

I was designing a boiler control where the temperature had to be
maintained just shy of boiling for a pharmaceutical application.  The
system was dangerous IMO (they took out the relief valves on the
theory that they could  harbor bacteria).

I switched SSR's on all three phases, dropping two phases when the
temperature got close to set-point.  The SSR's didn't switch the 3ph
directly, they worked three mercury contactors that switched the 30
amps for the heater.  One of the SSR's did eventually fail;  if a
second had failed it would have been catastrophic... luckily, an
observant operator noticed a difference in the way a circular chart
record of the daily heating cycle looked and called my attention to
it.  It was still meeting specification, but the chart just looked
different than it had; and things don't happen for no reason at all.

Re: Solid state relay questions
On Sun, 09 Aug 2020 15:05:40 -0700, wrote:

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Not sure what you are trying to do...  you don't already have power to
the pump? Depending on the system/pump if the pump is left off for a
prolonged time and the pressure bleeds off, the pump could lose it's


It is only necessary to use one SSR for the switching, the other side
of the AC line doesn't need to switch (necessarily for the operation -
safety is something else)

I ran one set of contacts on my well pump, when one set died, just to
keep it running until a replacement pressure switch could be found.

It would probably work, bear in mind you may want to provide a
heatsink for the relays, and SSRs do have some leakage current, not
enough to run the pump but enough to give a shock if there's no load
on them.

It pays to read-up on how to set the pressures on the switch and tank
charge port.  By minimizing "short-cycling" you increase efficiency
and prolong component life.

Some of the newer systems don't use pressure tanks or just enough of a
"tank" to provide switch action to shut the pump off when not being
used.  The theory is (or the propaganda is) that there's virtually no
savings to be gleaned from having a big bulky expensive tank.  But
that could just be some scammer trying to shave more from his
construction projects.

Re: Solid state relay questions

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   This is the way the power situation is now: Imagine the well being
on the 50 yard line of a football field. The house is at one end of
the field, the shop at the other. Power now comes from the shop. I
want to power the well from the house. If I run wires from the house
and then back to the well the calculated voltage drop will be
excessive. The length of the wire will be about 3 times what it is now
and the voltage drop as it is wired now is OK but can't tolerate much
more length.  
   But if I use the pressure switch at the shop to control another
switch at the well head then the voltage drop will be the same as it
is now.
  It is not a trivial matter to move the pressure tank, and the
pressure switch must be located at the pressure tank.
   I already have power running to the well head from the house so the
only thing I need to do to have the house power the well is to figure
out the switching at the well head.
   The current switch has two contacts and therefore switches both
legs of the 240 volts. I figured that since the existing pressure
switch switches both legs I should copy it. And, since SSRs can fail
closed then if I use two and one fails closed the well still wouldn't
stay on.
   But maybe I need to rethink  and use a sealed relay, as you advise.
I can still do everything else the same way.

Re: Solid state relay questions
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So you're setting up a remote pressure switch to control the pump. I guess
you'll have a DC source in the pump-house that powers the switch wire
and subsequently the solid-state relays.

If your pump does not use neutral you probably don't need to switch
both live conductors with relays. Circuit beakers interrupt both
wires because they are safety devices.

To confirm the sutability of those relays you need to know the stall
or stariong current of your pump motor. Your relay needs to meet or
exceed that requirement.

Another option would be to run a pipe back from the tank to the
wellhead and attach the pressure switch to that pipe. as there would be
no flow in the pipe the pressure reading would be accurate (except for
gravity effects if there a height difference) and a small diameter
pipe could be used: 1/8" for example.

Whatever you do be sure to maintain the ground bonding connection to
the pressure tank in your workshop especially if you are using metal


Re: Solid state relay questions
On Mon, 10 Aug 2020 00:08:29 -0000 (UTC), Jasen Betts

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I already have a remote switch. I just want to use it to switch a
relay and not a pump. The SSR I'm considering uses AC for the control
voltage. I could run a pipe as you suggest but this would require a
long trench. Besides, the switching arrangement has been working fine
for over 20 years. I'm just making changes because I need to supply
the well with power from the house so it can be generator powered when
the power goes out.

Re: Solid state relay questions wrote:
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I'm pretty sure you don't want this type of "zero voltage" SSR for running  
motors. It might work fine, but is a type that costs more and will result  
in higher inrush current for inductive loads. SSRs also tend to fail "on",  
so you may end up with a situation like one hot switched on at all times  
and the other leg still working. A plain old contactor will fail in a more  
predictable way.

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