To fuse equipment or not to?

[Fred]

I'm putting together a circuit board to take in 12V at less than 1/2 amp
from a wall wart. The unit will have a power jack to suit.

In this design a fuse will be very large or non-resettable or expensive. I
already have a diode so reverse polarity won't be a problem but what exactly
are the rules regarding fuses? Do I need one?

I'm in the UK.

 

[CWatters]

"Fred" <Fred@nospam.com> wrote in message
news:4242a5e8$0$32605$db0fefd9@news.zen.co.uk...

I'm putting together a circuit board to take in 12V at less than 1/2 amp
from a wall wart. The unit will have a power jack to suit.

In this design a fuse will be very large or non-resettable or expensive.
I
already have a diode so reverse polarity won't be a problem but what
exactly
are the rules regarding fuses? Do I need one?

I'm in the UK.

The wall wart should come CE approved/marked - which means it meets all the
necessary regulations for it's intended market place. That includes safety
and EMC standards. In which case the output should be intrinsically safe (eg
you should be able to touch the output safely or short the 12V output to 0V
without the wall wart catching fire or melting or presenting a shock
hazard). So you shouldn't need a fuse to protect the wall wart.

See Internal Fuses..
http://www.powerstream.com/Wall-mount-FAQ.htm

However... If the wall wart can be detached from your unit then you may
need to consider what happens if the user connects the wrong type of wall
wart to your product. Would your product catch fire? Other issues that might
be a factor....Will the wall wart have a functioning earth? (eg will the 0V
output be connected to the earth pin or will the wall wart be double
insulated and have no connection between 0V and earth?) If your product has
any power outlets (eg 5V on a USB port or PS2 keyboard port) then it may
make a difference. You might need a fuse on the input or elsewhere for some
of these reasons - but I'm too rusty on the rules for all this.

In the past when I was involved with product safety certification (8 years
ago now) we would hire someone at the BSI for an hour and take along a
prototype along for an informal meeting. (In Hemel Hempstead I think it
was). We'd take along a prototype and go over the design to see if they can
spot any issues we need to address before we get too far down the design
path. You might be able to get this initial meeting free if you explain you
haven't done this before and want to know what services the BSI can offer.

When it comes to selecting a wall wart don't just take the manufacturers
word for it when they say it's CE approved! Ask them to provide evidence of
compliance (eg copies of test reports). Batches of wall warts shipped to you
should also come with a certificate of conformance. Some of the relevant EN
standards are mentioned on this makers site:

http://www.manby.com/products/home.htm

Colin

 

[philo]

Fred wrote:

I'm putting together a circuit board to take in 12V at less than 1/2 amp
from a wall wart. The unit will have a power jack to suit.

In this design a fuse will be very large or non-resettable or expensive. I
already have a diode so reverse polarity won't be a problem but what exactly
are the rules regarding fuses? Do I need one?

I'm in the UK.



fuses are meant more for fire-protection than for protecting the

equiment itself...
if size is an issue, there are very small pico fuses avail

 

[JANA]

It is a good practice to use a fuse in any equipment. The wall wart supply
most likely has an internal fuse. Putting an external one that is a bit
smaller than the one in the wall wart, will save you extra inconvenience if
there is a short. Having an extra fuse in line adds to the safety factor as
well.

--


Greetings,

JANA
============

"Fred" <Fred@nospam.com> wrote in message
news:4242a5e8$0$32605$db0fefd9@news.zen.co.uk...

I'm putting together a circuit board to take in 12V at less than 1/2 amp
from a wall wart. The unit will have a power jack to suit.

In this design a fuse will be very large or non-resettable or expensive.
I
already have a diode so reverse polarity won't be a problem but what
exactly
are the rules regarding fuses? Do I need one?

I'm in the UK.

 

[Jim Gregory]

There should already be a fuse in your UK13-A plug, but I have often noticed
it was sometimes omitted (naughty!) from a monolithic 13-A plug format PSU
(you may have called it something else).
The fuse at the *correct* rating in a plug top is there just to protect the
consumer unit (fuse board) and then the *local mains supply*.
When that blows due to any number of failure reasons, the resultant cut-off
of maintained supply just to the appliance is pure spin-off.
Many consumer units now have resettable trips, a bit in action like RCDs.
Anti-Surge (T) fuses on the AC side to handle both inrush and normal current
to a AC to DC PSU (allow 20% above peak inrush) and Quick-Blow (F)
protection fuses (allow 10% above max traffic) after the o/p of a DC PSU
are recommended.


"Fred" <Fred@nospam.com> wrote in message
news:4242a5e8$0$32605$db0fefd9@news.zen.co.uk...

I'm putting together a circuit board to take in 12V at less than 1/2 amp
from a wall wart. The unit will have a power jack to suit.

In this design a fuse will be very large or non-resettable or expensive.
I already have a diode so reverse polarity won't be a problem but what
exactly are the rules regarding fuses? Do I need one?

I'm in the UK.