(Newbie) Fuses with MOV, thermal or PCB?

[Ant_Magma]

I'm looking for a fuse to protect my MOV which is to be placed across
the line voltage (240VAC, Live & Neutral). The MOV has a rating of
around 375V-390V.

L--|Fuse|---|-------| |
| |MOV |Protected|
| | |circuit |
N-----------|-------| |

Questions:
1) Since one of my components requires a current of around 500mA, i was
thinking a 1A fuse should work alright (or should i go for a 0.63mA
fuse?).

2) At Farnell, there are listed 2 types of fuses (among many others)
PCB and thermal fuses. Since i have close to zero practical experience,
which 1 should i be looking at?

3) These fuses are packaged with leads, does it mean whenever the fuse
blows i have to desolder and replace it?

 

[John Popelish]

I'm looking for a fuse to protect my MOV which is to be placed across
the line voltage (240VAC, Live & Neutral). The MOV has a rating of
around 375V-390V.

L--|Fuse|---|-------| |
| |MOV |Protected|
| | |circuit |
N-----------|-------| |

It is generally not practical to protect an MOV from damage, since
this occurs during moments when the line voltage is way above normal,
where the fuses are rated to open. What the fuse will do is isolate
the burnt out (shorted) MOV from the power line, after the voltage
surge that damaged it, has passed.

Questions:
1) Since one of my components requires a current of around 500mA, i was
thinking a 1A fuse should work alright (or should i go for a 0.63mA
fuse?).

You need to also be aware of any inrush current that charges up
capacitors.

2) At Farnell, there are listed 2 types of fuses (among many others)
PCB and thermal fuses. Since i have close to zero practical experience,
which 1 should i be looking at?

It depends on the inrush situation. Thermal lag fuses tolerate a
brief over current better (longer) than fast blow types do.

3) These fuses are packaged with leads, does it mean whenever the fuse
blows i have to desolder and replace it?

Yes. You will also probably have to replace whatever else shorted and
caused the fuse to blow (MOV, rectifiers, across the line capacitors,
after rectifier capacitors, or other loads). Having an MOV lowers the
likelihood that other things will be blown as a result of a line surge.

 

[Ant_Magma]

Thanks for your reply John.

You mentioned that it's not practical to place a fuse before a MOV
because by the time the fuse reacts and opens the circuit, the surge
would have already destroyed the MOV correct?

It's kinda confusing because in many papers they recommend using fuses
together with MOVs. Then what do you propose?

Or instead of placing the fuse before the MOV, i place the fuse after
the MOV? As a last like of defence in case the MOV fails?

How do i take into account the in rush current? Can you give some
examples? Such as for A amount of current, what type of fuse should i
use and what to do about the in rush current?

Thank you

 

[Rich Grise]

Thanks for your reply John.

You mentioned that it's not practical to place a fuse before a MOV
because by the time the fuse reacts and opens the circuit, the surge
would have already destroyed the MOV correct?

No, no! All he meant was that the fuse won't protect the MOV - it _does_,
however protect the house from burning down when the MOV goes short.

It's kinda confusing because in many papers they recommend using fuses
together with MOVs. Then what do you propose?

Use a fuse under any circumstances, whether you go with MOVs, transzorbs,
spark gaps, or hash chokes - you _always_ need a fuse.

Good Luck!
Rich

 

[John Popelish]

(snip)
Thanks for your reply John.

You mentioned that it's not practical to place a fuse before a MOV
because by the time the fuse reacts and opens the circuit, the surge
would have already destroyed the MOV correct?

Read it again. I said no such thing. I said that you should not
expect the fuse that is upstream of the MOV to protect it from damage
during by a high voltage surge.

But the fuse is needed to protect the wiring from the effect of a
damaged MOV, after the surge has passed.

It's kinda confusing because in many papers they recommend using fuses
together with MOVs. Then what do you propose?

Fuse upstream of the MOV, but don't expect it to save the MOV. It
will prevent your house from burning down after the MOV partly shorts
and sits there glowing red hot.

Or instead of placing the fuse before the MOV, i place the fuse after
the MOV? As a last like of defence in case the MOV fails?

How do i take into account the in rush current? Can you give some
examples? Such as for A amount of current, what type of fuse should i
use and what to do about the in rush current?

The best way is to measure it, but it can be calculated or simulated,
approximately, if you have an adequate model of the entire input
circuit.

There are also things you can do to reduce the inrush, including line
inductors, selecting a higher leakage inductance transformer, adding
one or more NTC thermistors in series, adding resistance in series,
etc., as applicable. These remedial actions may allow a faster
response fuse that will protect more components from damage. But the
real purpose of a fuse is to limit damage after something has failed.

The first step may be to measure what the surge current is for your
present design, when the line is connected at the peak voltage point
(worst for capacitive loads), and when turned on at the zero crossing,
with the incoming half cycle in the same direction as the last half
cycle before the line was previously disconnected (worst case for
transformer core saturation). Of you make lots of random phase
measurements and look for the worst one that happens by chance, and
hope you have seen close to the worst that can happen.