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Default 7 day timer and Inductance current


I have a 7 day timer, with a max load of 13A at 240V.
However reading the instructions its states there is an inductance
current of 3A. What does this mean ?

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Default 7 day timer and Inductance current

On 01/01/2014 12:54, Chris Crinkle wrote:

I have a 7 day timer, with a max load of 13A at 240V.
However reading the instructions its states there is an inductance
current of 3A. What does this mean ?

If the load isn't resistive like a heater element, then the permitted
load is reduced.

Inductive loads such as the motors used in fridges or washing machines
can quite easily generate sparks when the power is switched off that in
an etreme case can weld the contacts together or damage them over time
so that they no longer make a low resistance connection. The problem can
be minimised by bridging the contacts with a capacitor, but then you get
problems with the permanent leakage current into the load, with the
safety problems that can cause.

--
Tciao for Now!

John.
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Default 7 day timer and Inductance current

In article , Chris Crinkle
writes

I have a 7 day timer, with a max load of 13A at 240V.
However reading the instructions its states there is an inductance
current of 3A. What does this mean ?

It means that the 13A rating applies only to purely resistive (heating)
loads.

An inductive load is something like a motor or an old style (inductive
ballast) fluorescent tube lamp and when a circuit containing something
like that it disconnected, voltages are generated that could cause
arcing across the opening switch contacts. The inductive rating is the
maximum current that can be switched on that kind of load without
severely limiting the life of the contacts due to arc wear or indeed
welding them together.

Similar restrictions apply to loads with large inrush currents such as
computer power supplies.
--
fred
it's a ba-na-na . . . .
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Default 7 day timer and Inductance current

On 01/01/2014 12:54, Chris Crinkle wrote:

I have a 7 day timer, with a max load of 13A at 240V.
However reading the instructions its states there is an inductance
current of 3A. What does this mean ?


It means that if you are switching a load that is inductive in nature
rather than simply resistive, then the maximum capacity is reduced to
3A. (inductive loads cause big switching transients that have a habit of
welding the relay contacts in the programmer!)


--
Cheers,

John.

/================================================== ===============\
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Default 7 day timer and Inductance current


"Chris Crinkle" wrote in message
...

I have a 7 day timer, with a max load of 13A at 240V.
However reading the instructions its states there is an inductance current
of 3A. What does this mean ?


Inductive loads (motors, coils etc.) have stored energy in their coils which
means the current you have to break can be much higher than is on the rating
plate.

So any switch rated for a non-inductive load (lights, heater) will be down
rated for inductive loads.
Some are not suitable at all for inductive loads.




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Default 7 day timer and Inductance current

Yes you get a storage effect much like a magneto.

Brian

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From the Sofa of Brian Gaff Reply address is active
"John Williamson" wrote in message
...
On 01/01/2014 12:54, Chris Crinkle wrote:

I have a 7 day timer, with a max load of 13A at 240V.
However reading the instructions its states there is an inductance
current of 3A. What does this mean ?

If the load isn't resistive like a heater element, then the permitted load
is reduced.

Inductive loads such as the motors used in fridges or washing machines can
quite easily generate sparks when the power is switched off that in an
etreme case can weld the contacts together or damage them over time so
that they no longer make a low resistance connection. The problem can be
minimised by bridging the contacts with a capacitor, but then you get
problems with the permanent leakage current into the load, with the safety
problems that can cause.

--
Tciao for Now!

John.



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Default 7 day timer and Inductance current

On Thu, 02 Jan 2014 08:37:33 +0000, harryagain wrote:

"Chris Crinkle" wrote in message
...

I have a 7 day timer, with a max load of 13A at 240V.
However reading the instructions its states there is an inductance
current of 3A. What does this mean ?


Inductive loads (motors, coils etc.) have stored energy in their coils
which means the current you have to break can be much higher than is on
the rating plate.

So any switch rated for a non-inductive load (lights, heater) will be
down rated for inductive loads.
Some are not suitable at all for inductive loads.


I don't really understand why peeps here are describing heaters as non-
inductive loads. They most certainly ARE inductive; not so much as
motors, but significantly inductive nevertheless. Reason being the
resistance wire used in them is bunched up in coils to save space and
where you have a coil you have inductance. In fact even a straight wire
has a tiny amount of inductance but that's another story.
Coils store energy in the form of a magnetic field and when the current
is switched off, this field collapses generating a back EMF (a voltage
spike). This can be several thousand volts in some cases, causing arcing
of the switch terminals and shortening their life. You therefore have to
'de-rate' the current to allow for this.
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Default 7 day timer and Inductance current


"Cursitor Doom" wrote in message
...
On Thu, 02 Jan 2014 08:37:33 +0000, harryagain wrote:

"Chris Crinkle" wrote in message
...

I have a 7 day timer, with a max load of 13A at 240V.
However reading the instructions its states there is an inductance
current of 3A. What does this mean ?


Inductive loads (motors, coils etc.) have stored energy in their coils
which means the current you have to break can be much higher than is on
the rating plate.

So any switch rated for a non-inductive load (lights, heater) will be
down rated for inductive loads.
Some are not suitable at all for inductive loads.


I don't really understand why peeps here are describing heaters as non-
inductive loads. They most certainly ARE inductive; not so much as
motors, but significantly inductive nevertheless. Reason being the
resistance wire used in them is bunched up in coils to save space and
where you have a coil you have inductance. In fact even a straight wire
has a tiny amount of inductance but that's another story.
Coils store energy in the form of a magnetic field and when the current
is switched off, this field collapses generating a back EMF (a voltage
spike). This can be several thousand volts in some cases, causing arcing
of the switch terminals and shortening their life. You therefore have to
'de-rate' the current to allow for this.


The inductive effect is insignificant in electric heaters with a few dozen
turns of conductor in air.

Significant inductors have hundreds of turns of wire wound onto iron cores
which store significant energy in the form of a changing magnetic field.


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