New Zealand Fuses replaced with higher amperage. Thoughts?

[Sparkswillfly]

Background:
House was built early 1980s, fuse board is mostly the old school ceramic fuses. The wiring is said to be of modern design with 'tough plastic sheathing' according to a pre-purchase inspection.

The previous owner has replaced a 5 amp fuse for the lights with an 8 amp circuit breaker.The breaker has tripped whenever a bulb has blown but never at any other time. Most of the bulbs are incandescent but am upgrading to LED as each bulb blows. This circuit is for all 13 light fittings in the house. (its not like I'm ever likely to have every light in the house on at the same time either, there is only one of me) There is also one other 5 amp circuit although thats still on an old school fuse and I can't think what it would be for?

They have also replaced a 15 amp fuse with a 16 amp circuit breaker on one of the wall outlet circuits, but this one has never had a large load placed on it, at least not large enough to trip. I was looking at replacing all of the old ceramic ones with modern circuit breakers for ease of use and safety, but you can't seem to get 15 amp ones anymore just 16 amp ones. This perhaps explains the previous owner's actions.

I always thought you should never replace fuses with anything of a higher amperage, yet the previous owner has done just that in 2 instances.

1.How concerned do I need to be about these higher amp breakers? Is it an issue at all given the small difference and that the circuit is unlikely to be operated at maximum capacity?
2.Should I replace the other dozen old ceramic fuses with circuit breakers, bearing in mind the 15 amp ones might be replaced with 16 amp ones?
3.What could the other 5 amp circuit relate to if not lights?

Thanks

 

[telectrix]

fuses and breakers should be rated according to the cable/s used on their respective circuits. tese cables having been selected according to the size of the load. i.e. the cable rating for current capacity must be higher than the actual load current. the rating of the breaker must be less than the max. rating of the cable. ober here we use a simple formula
Ib<In<Iz, where Ib = deign current; In = MCB rating; Iz = cable max current rating.

so, if your cable is capable of 10A, a 8A breaker is correct.

 

[radiohead]

Assuming the original wiring supplied by the 15a fuse is standard 2.5mm or whatever is equivalent in your neck of the woods for a radial power circuit then a 16a mcb will be fine.
The unknown 5a could supply a bell transformer or something. One way of finding out if it's not obvious is to remove the fuse and wait until you notice something not working, obviously check essentials like fridges are still working.

 

[pc1966]

In most cases the nearest breaker to the fuse if fine, so in the UK where it was 5/15/30A fuses then you would see 6/16/32A MCB in their places. LED replacement lighting will use less power than the tungsten lamps so you should have plenty of capacity with your current rating.

Q1) You should not be worried by the small difference.

MCB have a different trip characteristic to a fuse and generally they are faster. The down-side of that is they are more likely to trip in the event of a fault as they may respond before the end-device's fuse has blown, but the advantage is generally less fault energy gets through.

Q2) If it were me I would replace with breakers, they are also much easier to reset.

Q3) As already said, it could be a bell transformer, old outdoor lights, alarm power, etc. You might be able to trace cables but pulling the fuse and looking to see what has stopped working is the easy way!

 

[westward10]

In the UK we apply a correction factor of 0.725 to semi-enclosed fuse elements so for a 15A element the cable must be rated at 20.7A. If the circuit has been designed in such a way replacing the 15A fuse with a 16A mcb has theoretically derrated the circuit.
For a 5A element you have 6.9A with the correction factor so an 8A mcb has slightly uprated the circuit.

 

[pc1966]

While it might not be exactly the same as your situation, here is a comparison of the break time versus current for a fuse (32A BS88 Gg type) in red, and a MCB (Schneider Acti9 iC60 32A B-curve) in blue. It shows at most overload currents the MCB is faster, most dramatically above the 100-150A region where the magnetic "instantaneous" trip takes over.

OK, will try later as it seems I can't upload the picture

 

[Sparkswillfly]

Thanks for the replies. Is an MCB on each circuit enough or is it better/safer to upgrade each one to an RCBO to provide protection against getting zapped?

 

[pc1966]

Thanks for the replies. Is an MCB on each circuit enough or is it better/safer to upgrade each one to an RCBO to provide protection against getting zapped?

Definitely the RCBO!

That gives you the ease of reset for overload (like the MCB) but also gives you protection against a low current live to earth fault (electric shock and fire risks from mice chewing cables, etc).

Many UK homes have 1 or 2 RCD followed by sets of MCB, and while that achieves the same functional goals it is less reliable than an all-RCBO setup in the sense that if there is a leakage fault on any of the downstream circuits from the RCD it trips, disconnecting all of those circuits, but you don't know which one was at fault!

 

[telectrix]

Definitely the RCBO!

That gives you the ease of reset for overload (like the MCB) but also gives you protection against a low current live to earth fault (electric shock and fire risks from mice chewing cables, etc).

Many UK homes have 1 or 2 RCD followed by sets of MCB, and while that achieves the same functional goals it is less reliable than an all-RCBO setup in the sense that if there is a leakage fault on any of the downstream circuits from the RCD it trips, disconnecting all of those circuits, but you don't know which one was at fault!

eeny meeny mineey mo. grab your tester and switch to Go.

 

[ipf]

eeny meeny mineey mo. grab your tester and switch to Go.

Makes it a bit more interesting, dunnit...

 

[pc1966]

While it might not be exactly the same as your situation, here is a comparison of the break time versus current for a fuse (32A BS88 Gg type) in red, and a MCB (Schneider Acti9 iC60 32A B-curve) in blue. It shows at most overload currents the MCB is faster, most dramatically above the 100-150A region where the magnetic "instantaneous" trip takes over.

OK, will try later as it seems I can't upload the picture

Attached is the comparison, with my crude attempts to fill in the bits not plotted by the Schneider on-line tool.

Basically as current increases a fuse blow faster (though it gets complicated and a bit variable down below one-two cycles of the AC supply as the time being split in to pre-arcing when the element is still solid, and arcing when it has vaporised and the arc is dying down) whereas a MCB when it gets to the "instant" magnetic trip it operates in 5-10 milliseconds that decreases, but not massively, with higher fault current.

So in this case you see the 32A MCB (blue curve) has lower fault energy up to around the 0.5-1kA region (especially just above 100A when the magnetic trip fires) and beyond that the fuse (red curve) blows faster and is also far more effective at limiting the peak fault current / total let-through energy.

That is why it is still very common to see boards of breakers backed up by HRC fuses - the breakers have excellent behaviour at low/medium fault currents and can be reset quickly, whereas the fuses can sit there for decades and only blow under serious faults, but very quickly limit the energy so the breaker, busbars, etc, suffer much less risk of damage.