basic electrical questions

[jamesharrison]

Hi all

Can anyone please ask me some basic electric questions:


1) an over sized fuse is dangerous as it could cause damage to appliance(s) and fire?

2) an under sized fuse will not allow sufficient current through and will blow and appliance etc not work?

3) what are volt free contacts ie in a programmer/thermostat?

4) an oversized wire ie 2.5mm used on a lighting circuit is safe, but an undersized wire ie 1.5mm used on a ring main circuit is dangerous, as could overheat and cause fire

5) using a pull-cord switch that has 45 amps 230 volts on it is ok/safe to use on a circuit which uses say 30 amps 230 volts?

if anyone can quickly answer these I would be very thankful.


James

 

[riveralt]

Hi all

Can anyone please ask me some basic electric questions:

1) an over sized fuse is dangerous as it could cause damage to appliance(s) and fire?

The purpose of the fuse is to blow before the cable exceeds it current carrying capacity.

2) an under sized fuse will not allow sufficient current through and will blow and appliance etc not work?

see answer to 1

3) what are volt free contacts ie in a programmer/thermostat?

?

4) an oversized wire ie 2.5mm used on a lighting circuit is safe,

yes but you will have big problems fitting the cable into the ceiling rose terminals.

but an undersized wire ie 1.5mm used on a ring main circuit is dangerous, as could overheat and cause fire

Yes it could.

5) using a pull-cord switch that has 45 amps 230 volts on it is ok/safe to use on a circuit which uses say 30 amps 230 volts?

Yes

 

[echoes]

3) what are volt free contacts ie in a programmer/thermostat?

They are contacts which are not connected to any voltage internal to the programmer or thermostat - like relay contacts. Can therefore be used with mains voltage or any other low voltage, depending on what they are controlling.

 

[bernardgreen]

Hi all

Can anyone please ask me some basic electric questions:


1) an over sized fuse is dangerous as it could cause damage to appliance(s) and fire?

2) an under sized fuse will not allow sufficient current through and will blow and appliance etc not work?

3) what are volt free contacts ie in a programmer/thermostat?

4) an oversized wire ie 2.5mm used on a lighting circuit is safe, but an undersized wire ie 1.5mm used on a ring main circuit is dangerous, as could overheat and cause fire

5) using a pull-cord switch that has 45 amps 230 volts on it is ok/safe to use on a circuit which uses say 30 amps 230 volts?

[1] Fuses do not normally cause damage, they limit the amount of damage that a fault can cause.

[2] Correct. But a fuse that is under sized can glow red hot for some time before melting. That may damage the fuse holder

[3] volt free contacts are contacts that have no electrical connection to any other ciruit in the unit. ( such as a timer or electronic thermostat )

[4] undersized wire ( copper too thin ) is only dangerous if the fuse allows enough current to flow to heat the copper above a safe operating temperature. Undersized insulation is always dangerous.

[5] provided the cables are the right size and properly install it should be safe

these sound like exam questions.

 

[PrenticeBoyofDerry]

Hi all

Can anyone please ask me some basic electric questions:


1) an over sized fuse is dangerous as it could cause damage to appliance(s) and fire?

Yes the fuse rating must be less than the current carrying capacity of the cable, taking any derating factors in to consideration (ie. cable routes through insulation and conduit) as the fuse/protective device must break before the cable is allowed to over heat and melt.

2) an under sized fuse will not allow sufficient current through and will blow and appliance etc not work?

If enough current is not offered to the circuit this could cause overloading of the circuit.

3) what are volt free contacts ie in a programmer/thermostat?

They have no electrical connection to any other circuit

4) an oversized wire ie 2.5mm used on a lighting circuit is safe, but an undersized wire ie 1.5mm used on a ring main circuit is dangerous, as could overheat and cause fire

It is safe to use a cable CSA that is larger than the one needed, but as pointed out, specially on lighting circuits, terminating the conductors can often be difficult, as there is normally a restricted amount of room in the terminals.

5) using a pull-cord switch that has 45 amps 230 volts on it is ok/safe to use on a circuit which uses say 30 amps 230 volts?

As the rating of the accessory is higher than the current being permitted through the circuit, this is perfectly safe, if it was the other way around that could cause problems regarding the accessory terminals over heating.

 

[jamesharrison]

Thanks for your kind replies

They are just some questions have been thinking of over the pasty few years.

I am replacing an immersion heater timer in my house, there's power to it but not from it to the heater element. Bascially the one I am looking at says it is 16 amps and 230 volts - basically my heater runs of a 15 amp fuse - so the 16 amp timer, is that the maximum it can take, so suitable for mine?

 

[holmslaw]

..

 

[bernardgreen]

Sounds right, with electricty switches etc with a higher current rating than required will always be ok.

But NOT fuses, MCB and similar protective over current devices.

Playing devils advocate a magnetically self latching switch might not hold on with current lower than its rated current ( after all they are design to switch off when the current drops ) but I doubt a DIYer would ever be using one.