earth loop impedance

[plugwash]

yes it should..
discrimination..

a fault should take out the first over current device it comes to in a circuit.. otherwise whats's the point having it..?

unfortunately that went out the window with the introduction of MCBs. a B32 will very often trip before a 13A plug fuse under short cuircuit (heck i've seen a B16 go before a 3A plug fuse!).

the plug fuse is still needed for overload protection though.

 

[tim west]

yes it should..
discrimination..

a fault should take out the first over current device it comes to in a circuit.. otherwise whats's the point having it..?

unfortunately that went out the window with the introduction of MCBs. a B32 will very often trip before a 13A plug fuse under short cuircuit (heck i've seen a B16 go before a 3A plug fuse!).

the plug fuse is still needed for overload protection though.

I think ColJack was making the point in mentioning discrimination that the design of the circuit should take into consideration the allocation of protective devices in accordance with their position in a circuit and by the tables in the regs for selection of said devices, this should take care of devices in the fixed wiring which the regs appertain to, the plug top fuse not being part of the fixed installation doesn't come into the calculations when designing the circuit. If you think about it ,it could be anything plugged into that socket therefore how could it become part of the design of the circuit? so to say Discrimination has "gone out the window" is a rather bliase statement.

 

[plugwash]

even if you don't count the fuses in plugs because they are not part of the fixed installation (which is imo a serious cop-out) all those fuses in FCUs suffer the exact same issue.

the fact is that if you put a fuse downstream of a MCB unless there is a VERY large ratio (far more than 2x) between the two then the magnetic trip on the MCB is very likely to cut in

another way in which descrimination has gone out of the window in recent times has been RCDs. it is very common to put a single rcd over the whole house or at least everything except the lighting cuircuits. A fault to earth with anyappliance in the house is virtually gauranteed to to trip the rcd before it trips the more local overcurrent protection device.

In a typical domestic environment (no sub boards) the only place where discrimination is still happening correctly is between the breakers in the CU and the service fuse, everywhere else it is shot to hell because of the introduction of MCBs and RCDs at the CU while leaving fuses as the downstream protection.

 

[scousespark]

another way in which descrimination has gone out of the window in recent times has been RCDs. it is very common to put a single rcd over the whole house or at least everything except the lighting cuircuits. A fault to earth with anyappliance in the house is virtually gauranteed to to trip the rcd before it trips the more local overcurrent protection device.

an earth fault won't trip an MCB, unless it's part of one of those pesky devices which John loves so much

 

[RF Lighting]

an earth fault won't trip an MCB, unless it's part of one of those pesky devices which John loves so much

A dead short live to earth will definatly trip an MCB (Zs permitting)

 

[tim west]

even if you don't count the fuses in plugs because they are not part of the fixed installation (which is imo a serious cop-out) all those fuses in FCUs suffer the exact same issue.

the fact is that if you put a fuse downstream of a MCB unless there is a VERY large ratio (far more than 2x) between the two then the magnetic trip on the MCB is very likely to cut in

another way in which descrimination has gone out of the window in recent times has been RCDs. it is very common to put a single rcd over the whole house or at least everything except the lighting cuircuits. A fault to earth with anyappliance in the house is virtually gauranteed to to trip the rcd before it trips the more local overcurrent protection device.

In a typical domestic environment (no sub boards) the only place where discrimination is still happening correctly is between the breakers in the CU and the service fuse, everywhere else it is shot to hell because of the introduction of MCBs and RCDs at the CU while leaving fuses as the downstream protection.

I think you are missing the point here, Discrimination IS the act of using the tables when designing circuits, to prevent the very thing you are saying, A well designed circuit will use discrimination to prevent MCB's and Fuses interrupting the circuit where they shouldnt and if that means not mixing the two types in a particular situation then so be it. It doesn't mean that discrimination has become redundant in any way, far from it.

 

[tim west]

System playing up- duplicate post please remove

 

[Spark123]

Not redundant, just doesn't work!! If you look at the current required to interrupt a 13A plugtop fuse in 0.1s it equates to somewhere around 100A. The magnetic trip of a type B 32A MCB operates at between 3-5 x its rating hence somewhere between 96A and 160A can cause it to trip.

 

[securespark]

I must admit I was surprised to learn the max Zs for 1362's...

Max Zs for 1362 fuses



Fuse Rating.....0.4s........................5s.

2A...............43.4 ohms..............46 ohms
3A...............17.7 ohms..............23 ohms
5A...............19.2 ohms..............25 ohms
7A...............6.57 ohms..............10 ohms
10A.............3.97 ohms................6 ohms
13A.............2.53 ohms................4 ohms

 

[Spark123]

I can see where you got the info from Secure, but why is 3A max Zs lower than 5A??

 

[plugwash]

even if you don't count the fuses in plugs because they are not part of the fixed installation (which is imo a serious cop-out) all those fuses in FCUs suffer the exact same issue.

the fact is that if you put a fuse downstream of a MCB unless there is a VERY large ratio (far more than 2x) between the two then the magnetic trip on the MCB is very likely to cut in

another way in which descrimination has gone out of the window in recent times has been RCDs. it is very common to put a single rcd over the whole house or at least everything except the lighting cuircuits. A fault to earth with anyappliance in the house is virtually gauranteed to to trip the rcd before it trips the more local overcurrent protection device.

In a typical domestic environment (no sub boards) the only place where discrimination is still happening correctly is between the breakers in the CU and the service fuse, everywhere else it is shot to hell because of the introduction of MCBs and RCDs at the CU while leaving fuses as the downstream protection.

I think you are missing the point here, Discrimination IS the act of using the tables when designing circuits, to prevent the very thing you are saying

the point is the only way to achive that for socket cuircuits is not to use MCBs to protect them (given that the appliances connected to them whether fixed-wire or portable are protected by plug fuses) yet everyone electricians and diyers alike puts them on MCBs anyway!

modern breakers are very very fast, the 0.1s is a maximum defined in the standard and some manufacturers (read wylex's catalog sometime) even claim sub-cycle disconnect. As i said even with a factor of over 5 difference (3A plug fuse VS B16 breaker) in rated current the breaker can trip before the fuse blows!

likewise with typical domestic use of RCDs a single faulty appliance can easilly trip out every socket in the house thats hardly proper consideration of discromination.

 

[tim west]

the point is the only way to achive that for socket cuircuits is not to use MCBs to protect them (given that the appliances connected to them whether fixed-wire or portable are protected by plug fuses)

Again i stress the point that plugtops and their fuses do not form part of a final sub circuit as defined by the regs.
A fixed spur would be the exception,either the final sub circuit device mcb would be higher rated(to allow discrimination) which means the cable downstream would have to be rated as such, probably not practical, or the spur fuse is downrated accordingly to achieve discrimination, ie the design of a spur off circuit if fed say off a ring circuit may only be able to supply a 10A circuit or a max fuse rating in the spur of 10A this would have to be worked out using the regs tables and possibly also manufacturer's tables to decide on the proper protective devices.
"Hang on a minute" I hear you say "that is also impractical, suppose you want a spur to deliver 13A?"
Well the answer to that is, design of circuit is important here as a fixed spur is part of the installation, you wouldn't want to consider running say an immersion heater off a spur from a ring circuit would you? well I for one wouldn't, I'd make sure the fused spur was fed on it's own circuit from the CU and the circuit protected accordingly, this is discrimination applied in practice.

So this is what I meant by you being bliase with your statement, and that it's not discrimination that's gone out the window it's the application of circuit design by some that has.

yet everyone electricians and diyers alike puts them on MCBs anyway!

Again a bliase statement, not all do so, those that do are not applying thought to circuit design beforehand.
I come across a lot of "rule of thumb" attitude amongst some where statements like "stick it on an MCB that'll protect it better" or "stick it on an RCD and you can't go wrong" seem to be the norm. You cannot go into any form of electrical work with such a carefree attitude, as an electrician one must justify what components are used in a circuit and not accept the "general concensus" that an MCB or RCD is necessarily the best choice in any particular situation.

 

[dingbat]

Sorry, I just have to ask... what does "bliase" mean? I can't find this word in any dictionary, on the shelves or online.

 

[securespark]

I can see where you got the info from Secure, but why is 3A max Zs lower than 5A??

Might be typo, or an error from my source...

A nice guy on the IET forum gave them to me, but now I can't find them...


EDIT: I can now!!

Found the Bussman figures and they are correct.

There are two points: the 13A figures come from 7671.

The figures you queried come from "the time/current zones in BS1362".

 

[Spark123]

Spot on thanks. I take the 2A, 5A, 7A and 10A max Zs figures are only applicable to the bussman brand fuses??
Anyone got access to a copy of BS1362?