# Faulty relay

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I have a light that refuses to go on.

The light switch (wall type) is connected to a relay. Now the relay won't
"click". It used to work a couple of days ago. The relay has operating voltage of
24V which switches a 240V line (lights).

I would like to test (using a voltmeter) where I'm having problems. How do I
measure if 24V is going through the coil/magnet.

The relay box looks like this (wires go to points A,B,C,D):

A    24V     b
-------------
|            |
|            |
|            |
|            |
|            |
C------------D
240V

When the switch is off volmeter shows 240V between C and D

When the switch is on volmeter shows 0V between C and D

How do I measure voltage in A and B?

## Re: Faulty relay

On Sat, 17 Nov 2007 19:44:38 +0200, Jimmy Neutron

Depends on how it's wired.
If the switch is interrupting power to the relay then when
switch is off, both A and B would read close to 0V relative
to system ground.  If the switch interrupts relay to ground
connection then there would be 24V at power into the switch,
either A or B, less at the other B or A also close to 24V
relative to ground since there is no current flowing.

When the switch is off there should be 0V between A and B,
and when the switch is on there should be a voltage across A
and B... what that voltage is depends on what else is in the
circuit and the resistance of the relay coil.

When the switch is on there would be 24V at the high side (A
or B) and a lesser voltage at the low side (B or A).

This doesn't necessarily tell you whether the relay is
working  because having power through the coil is only the
electro part of an electro-mechanical relay.  Another test
might have been confirming that the switch itself is
functional by whether has continuity when switched on but
this needs not happen because...

The better test is what you have already done, that with the
switch off the relay contacts are open resulting in the 240V
between C and D, and when the switch is on and there is 0V
between C and D, it means the relay has completed, closed
the circuit at this point and that the fault is elsewhere.
I can't account for it not clicking anymore but this change
in voltage state is a sign it appears to be working.

Since you have 240V at the relay when the switch is on, it
is time to measure voltage between the hot and ground at the
light.  If you measure 240V the light is bad.  If you don't
then the contacts or wire from the relay to the light is
that I am not seeing because for it to measure 240V between
C and D when the switch is off, it must mean either C or D
is leading to ground, completing the circuit which would not
normally be the case if the wiring or contacts to the light,
or the light itself were broken.  Perhaps you have a high
resistance from a bad contact on the 240V loop in the
circuit and it is limiting current to the light?

## Re: Faulty relay

I suspect either C or D is ground.

I have a known ground wire nearby. What If I connect my voltmeter probes to from
ground to C or D to find which is the hot wire?

Should I then try the same and connect either A or B with my voltmeter probes?

This would be to find out how the wiring is probably set and lead me to find
where why the lights do not turn on.

## Re: Faulty relay

OK I tried it and ground and either c or d shows 240 so it's definitely the hot
wire going there.

A or be and ground shows 0 V either on or off.

## Re: Faulty relay

Sounds like you`ve got no 24 volts.

## Re: Faulty relay

On Sat, 17 Nov 2007 22:44:49 +0200, Jimmy Neutron

Forget about A and B from now on.  Since you have 240V at
both C and D (unless you have misinterpreted the relay
markings but I would think it very obvious since C and D are
heavier current carrying wires, usually), having 240V at
both means the relay is working.

Now turn off the switch and measure between ground and C,
then ground and D.  One will now be 240V and the other
won't.

What kind of light is this (bulb type)?  If incandescent,
measure resistance between the C or D contact which is not
240V, and ground, while the switch is off.  If some other
type of light, measure resistance between the relay C or D
contact which is not 240V, and the hot wire on the lighting
fixture.  Actually, you can probably skip doing this and
just go to the lighting fixture and measure between and
ground there.

If there is 240V between these two, it seems the light or
some part of it's internal wiring is bad.  If there is not
240V there, you have some bad contact or break in the wiring
between the relay and that point - so be careful, a break
could mean something you touch is hot so let's hope you have
grounded metal boxes or pipes if they are metal and proceed
with caution if this latter case is true.

It would be safer to turn off the power to this AC circuit
and just probe with a multimeter for continuity and
resistance, but a bit of a pain if the relay is far enough
away from the light that you'd need a jumper wire to read
the continuity and resistance between the relay end and the
light end of the connecting wire.  Actually "safer" is the
wrong term, it is safe either way done correctly but
mistakes are quite a bit more significant when 240V instead
of nearly 0V meter power.

## Re: Faulty relay

On Sat, 17 Nov 2007 22:29:27 +0200, Jimmy Neutron

It's possible, but again depends on how it's wired.
It "could" be wired such that the power goes to the light
then the relay breaks the ground connection, making one C or
D on the relay connected to ground, but it is more likely
the power goes to the relay, through it, and then grounds
through the light in which case neither of the relay C or D
contacts is ground.  How is this assembly constructed,
perhaps in a grounded metal box?

Yes you could do that.

Yes

How much current does this light assembly use and what is
the current rating on your multimeter?  If the light current
is less than the meter current (and you are very careful
including turning the meter back to voltage mode when you
are finished to prevent accidents) then you could set the
multimeter to current measurement mode and touch each probe
to the C and D contacts, thus completing the circuit without
the relay in it... at which point the light should light up
if it and the wiring between the relay and it are good.

If you do this, just be careful and remember to move the
multimeter probes in the meter sockets and/or put the meter
back in voltage measurement mode, as I have heard of far too
many people who forgot to do so and if you did, it would at
least blow a fuse if not worse.

## Re: Faulty relay

Jimmy Neutron wrote:

Your description reads as though the relay is working properly and the
problem is elsewhere. If the relay is truly a SPST as it must be with
the pinout you show then _is_ activating with the 24V control as
expected otherwise the voltage between C and D would not be changing. It
really doesn't matter which side of the load the relay is located -- in
a series circuit it is all the same. But one would measure the voltage
between A and B in exactly the same way it is measured between C and D
-- there is just 1/10th as much of it but it definitely should be readable.

--
John McGaw
[Knoxville, TN, USA]
http://johnmcgaw.com