Linking Heaters Together?

[xxBarneyxx]

Its no different to using a multiway extension lead is it? That only has one fuse in its plug

You have one fuse in the plug but then every plug that is plugged into the extension has its own fuse as well. Yes the MCB should kick in if anything goes wrong but personally when it comes to things that can start fires/zap people I prefer to have a back up system (fused plugs).

 

[Murphys-Law]

Both Rooster and Dark Entity were being replied to. IMHO it is not "Unbelievably Bad nor is it Bad Bad Bad" These things are BS Certified and very neat and usable. They are great for Pooters, Hi Fi and Tanks. I can see why people will throw their arms up in the air about water and electricity but the truth is that it isn't as bad as you might think (Salt Water is a different matter I will agree) But if one of your pumps goes "Bad" or anything really the Box will trip and you will be stuffed anyway. The people who write the electrical regs agree and next year they are changing things so that effectively all circuits will be on a MCB (GFCI for US readers) so you may well find you house (or half of it) in total darkness) if anything anywhere goes wrong.


Fuses in plug tops are not designed to protect the device on the end of the cable - They are bizarrely designed to protect the cable feeding the device - That is why there are only two BS recognised fuses - 3 amp and 13 amp - I know you can buy 5 amp ones but they are now not BS - The problem is that none of these fuses will blow faster than the MCB can operate.


Have a look at the stuff you use in your aquariums - How much of it ha an earth wire ? I have actually been in a situation where a heater was broken - An Oscar had Crunched it and all I felt was a slight tingle - The fish would have felt nothing - Water is a fairly good insulator, glass is even better and the stand was wood, where is the electricity going to go? - Through me!!! I was on a concrete floor (So well earthed) but it didn't really affect me

 

[Rooster]

Its no different to using a multiway extension lead is it? That only has one fuse in its plug

You have one fuse in the plug but then every plug that is plugged into the extension has its own fuse as well. Yes the MCB should kick in if anything goes wrong but personally when it comes to things that can start fires/zap people I prefer to have a back up system (fused plugs).

Quite right.... each item has a fuse for a reason... and thats only ONE reason - to protect its cable from overheating !

Maths lesson:
Fuses are calculated (or rather "should" be !) to blow at a current slightly above normal operating current... example:
100W heater, working on uk mains voltage of 240vac (I know this is arbitrary, but stick with me)
The calculation for working out the current is:
Current = Watts divided by voltage, so
A = 100/240 ("A" is the symbol for current)
A = 0.41666 Amps

Or nearly half an Amp... so we really ought to use a fuse of, say, 0.75A but standard fuses made for 3 pin plugs are commonly 3,5,13A there are others, but these are easy to find !

So we select the smallest available - that is larger than the value we calculated - in this case a 3A fuse is more than sufficient

If a fault occurs in the heater, and it goes "open circuit" then NO current will flow - it just wont work.
If it goes total "short circuit" then a large current will flow, causing the weakest link in the chain to burn out... hopefully the fuse in the plug, if not the next weakest link would be the fuse in your fuse box (circuit breaker in the distribution panel) and the power is disconnected.
Scary part is when the fault is not a short circuit, but a low resistance fault, which allows a large current - but not so large as to blow the fuse in the panel... invariably a 15A breaker, which will take a relitively long time to "blow" (or "trip")... so imagine this fault allows a 10A current, NO fuses will blow - so what is this current doing ? Heating up the wiring between the fault and the power station ! Now the wiring upto your house is sturdy stuff, hundreds of Amps before a problem... in house, the wiring may be ok upto 50A, but the wire fitted to the heater is probably only rated at 3 or 6 A ... so wheres the weakest link ?
Thats right, this cable is going to get hotter and hotter until it fails, or BURNS out.

This is why EVERY item MUST have its own fuse ! And correctly rated at that - dont just rely on that 13A brown one that came fitted to the plug !

An RCD wouldnt help in this case, because they rely on current going via a path "outside" the live and neutral circuit... such as flowing to earth.

Hope this clarifies the issue a tad !

 

[Murphys-Law]

To further add to the confusion a fuse is designed to carry its rated current indefinitely, it won't operate till the current gets to 1.4 times its rated capacity, and, then it may take a long time to eventually melt. They do not conveniently work in a nice ordered way down the line that they are fitted in. It is a known problem and called discrimination (or rather the lack of) Circuit breakers also have a heating element in them to protect against continuous overcurrent.


Most of the rest of the world use unfused plugs and rely typically on 16 or 20amp circuit breakers with groups of circuits protected by a RCD device. They are not dragging out their dead each morning and rows of houses are not in flames each day. I think we over do it and are mentally programmed to rely on a form of protection that isn't likely to ever do its job. There is a lot of information out there but to put it simply a total short on a cable will let a current flow that is proportional to the impedance of that circuit in question (The PSCC -Prospective Short Circuit Current) on the main feed to your house may well be in the thousands of amps and multiply this by time and you have an energy calculation that will show how hot the cable will become. Once you get further down the line towards your socket outlets and equipment the impedance of the circuit increases and the PSCC falls off dramatically - In a worst case scenario the main (House) fuse may never blow but the internal cabling would get rather hot! It will only take a few 10ths of an ohm for this to happen - Typically in a home the circuit protective devices are rated to be able to switch 6000 Amps but if you are near to your transformer this may be inadequate and you could need to fit 10000 amp devices.

Tests have been conducted in the past and the accepted current a healthy adult can survive was decided to be 30mA If you stick a pair of wires into drinking water a yard apart you cannot get this much current to flow - So to get a 3 amp fuse to blow you would be looking at an energy discharge of almost 1KW before the fuse operated ( 3 * 1.4 =4.2A * 240V = 1008W) In the case of fixed equipment the current time allowed for this to happen is 5 seconds (scary eh?)

To surmise - If you get an electrical fault in your tank it is unlikely that the fuse/s will blow it is likely that the Circuit breaker or RCD will operate first and the whole circuit will be off anyway - It is known that a 13 amp fuse will not operate before a 32amp mcb will operate (their time curves are different) and frequently a MCB will even beat a 3 amp fuse too.

HTH