How Fast Should A Fuse Trip If Cannot Let Go?

[Andy™]

livestock is much more suscepible than humans, mainly due the distance between the front and back feet

remember all those horses that died a few years back due to a faulty underground cable?

 

[SBS Dave]

Sh*t!!.  That's what old age does to you.

Been off the tools for 8 years. Stopped assessing electricians for NAPIT 2 years ago.

Don't even have a copy of the Big Green Book, let alone Guidance Note 7.

I am a city lad, so what do I know about livestock.

That's why a made a pig's ear of the post. I need to beef up my knowledge of the current Regs, or it will be like a lamb to the slaughter for me on the Forum.

This is an issue I won't duck, otherwise you might think I am chicken. So, I won't horse around anymore and I am looking for someone to bale me out.

Please quote me the relevant section of GN& page 44, or better still, scan me the page, so I can have a gander.

I know what you are thinking  -  This is a load of bullocks.

SBS Dave ( with egg on my face)

 

[steptoe]

had a 4293 today that went in 193mS  ,  close. 

 

[batty]

had a 4293 today that went in 193mS  ,  close. 

Sounds like it was designed to speck probably never been tested since it was put in.

 

[alextrician]

Does anyone know how fast a fuse or circuit breaker must disconnect if someone is receiving a shock and cannot let go?

At approx 13-15mA it causes involuntary muscle contraction and may prevent someone from letting go.

hello there.

a fuse or an MCB will allways disconnect in case of a fault current. in circuits not exceding 32A - 0.4 sec and 5 sec in circuits excedind 32A (and these values are for circuirs supplied at 1210-230V)( table 41.1). MCB's or fuses are not design to disconect in case of electric shock.

an RCD is design to be installed in the circuit as additional protection where the risk of shock exist; ei: socket outlet with rated current not exceding 20A intended to be used by ordinary people and intended for general use  and mobile equipment with rating not exceding 32A for use outdoor. Basiclly all sockets outlet in domestic properties must be RCD. 

there is the best of both world (RCBO) wich combines the properties of fuses and RCD's

 

[steptoe]

hello there.

a fuse or an MCB will allways disconnect in case of a fault current. in circuits not exceding 32A - 0.4 sec and 5 sec in circuits excedind 32A (and these values are for circuirs supplied at 1210-230V)( table 41.1). MCB's or fuses are not design to disconect in case of electric shock.

an RCD is design to be installed in the circuit as additional protection where the risk of shock exist; ei: socket outlet with rated current not exceding 20A intended to be used by ordinary people and intended for general use  and mobile equipment with rating not exceding 32A for use outdoor. Basiclly all sockets outlet in domestic properties must be RCD. 

there is the best of both world (RCBO) wich combines the properties of fuses and RCD's

that is incorrect,

yet another of the myths within our industry,

as for MCBs or fuses not being designed to disconnect in case of electric shock,  :slap   where on earth did you get that from? or are you saying that all circuits installed without RCD protection are in fact lethal and must be isolated immediately?  that'll be probably over 80% of the country then with no electric.

:shakehead

 

[msb2003]

hello there.

a fuse or an MCB will allways disconnect in case of a fault current. in circuits not exceding 32A - 0.4 sec and 5 sec in circuits excedind 32A (and these values are for circuirs supplied at 1210-230V)( table 41.1). MCB's or fuses are not design to disconect in case of electric shock.

an RCD is design to be installed in the circuit as additional protection where the risk of shock exist; ei: socket outlet with rated current not exceding 20A intended to be used by ordinary people and intended for general use  and mobile equipment with rating not exceding 32A for use outdoor. Basiclly all sockets outlet in domestic properties must be RCD. 

there is the best of both world (RCBO) wich combines the properties of fuses and RCD's

The regulations do specify 0.4 sec and 5 sec respectively, for TN systems and 0.2 and 1 sec for TT systems (don`t forget distribution circuits are the same as circuits >32A) however your comment I`ve highlighted that they will always disconnect in these times is incorrect. These are the maximum times permitted and they will only be achieved if your Zs values are complient with the values given (use the OSG values as the Regs values must have the rule of thumb applied and multiplied by 0.8).

If you cannot achieve the required Zs values, an RCD can be used

 

[steptoe]

The regulations do specify 0.4 sec and 5 sec respectively, for TN systems and 0.2 and 1 sec for TT systems (don`t forget distribution circuits are the same as circuits >32A) however your comment I`ve highlighted that they will always disconnect in these times is incorrect. These are the maximum times permitted and they will only be achieved if your Zs values are complient with the values given (use the OSG values as the Regs values must have the rule of thumb applied and multiplied by 0.8).

If you cannot achieve the required Zs values, an RCD can be used

is that a regulation?

or simply another misnomer?

:C

Ive yet to read a reg that states that you can ignore max Zs and install an RCD,

a lot of certain other criteria would need to be met first.

 

[alextrician]

I apologise for posting replies that are not 100% accurate.

It's still no "light" for me. The question still is: will a fuse/MCB disconnect in case of electric shock, assuming the Zs value are in the right parameters.

I should of start my own topic....sorry peeps, 

 

[NozSpark]

They won't disconnect due to the high resistance of your body to earth..

 

[alextrician]

Hmm, so they will not disconnect in case of electric shock...is that because high resistance of the body and they are not design to do so. They are working on thermal effect/magnetism. The current passing through, is not enough to melt the fuse element. Thre body resistance is about 500 ohms in average (300-1000 ohms depending on current path, gender, and body structute, weight, etc). A simple ohm's law shows that current passingthrough is about 0.46 ohms (I=V/R).

They will disconnect the supply in case of a Earth Fault Current in the designated time, assuming that the Ze value is in parameters stated by BS7671. Also the Ze values in BS7671 are design values, therefore a thermal constraint must be added (x0.8) or OSG values. Electric shock and Earth fault current are two different things dasically there is no risk of electric shock when a earth fault current occurs if no one is around to tuch it

And to answer Steptoe, the electrical isntallations are absolutely safe without the protection of an RCD, considering that no one will stick two wires in the socket and toch the ends. The RCD is a aditional protection in case that person wants to stick two wires in the socket...lol. What is the point of an residual device in the installation if the fuses/MCB's disconnecs the supply in case of electric shock?

Please corect me if I am wrong

 

[NozSpark]

The MCBs and fuses are only there to protect the circuit and will only operate if a properly designed circuit is overloaded; a L-N short is effectively an overload....

RCDs are additional protection to protect livestock and against fire.. they do this by detecting an imbalance in the L and N currents (earth fault) ; now this earth fault can be for many reasons.... a nail through a cable, a faulty appliance, you sticking a fork in the toaster to retrieve sone toast, etc.

Hope that clears things up

 

[alextrician]

Yes, course it makes sens, this is what I wanted to explain. I didn't know an RCD protects against fire caused by faults in the electrical installations. I thought that this job belongs go fuses and MCB's, you learn something new every day...lol

Thankx very much peeps,

 

[msb2003]

is that a regulation?

or simply another misnomer?

:C

Ive yet to read a reg that states that you can ignore max Zs and install an RCD,

a lot of certain other criteria would need to be met first.

Regulation 411.4.9 - Where an RCD is used to satisfy the requirements of regulation 411.3.2.2 the maximum values of earth fault loop impedance in Table 41.5 may be applied for non-delayed RCD`s

Regulation 411.3.2.2 gives disconnection times and table 41.5 gives maximum earth fault loop impedance values for RCD`s, 30mA 1667, 100mA 500, 300 mA 167, 500mA 100

Yes, course it makes sens, this is what I wanted to explain. I didn't know an RCD protects against fire caused by faults in the electrical installations. I thought that this job belongs go fuses and MCB's, you learn something new every day...lol

Thankx very much peeps,

A simple calculation explains the RCD protecting against fire, Power = Current x Voltage,     300mA x 230V = 69 Watts getting hot enough to cause a fire!!

 

[binky]

if you need to RCd circuit to meet Zs, I think I would want to be doing something about the circuit. LOL. Had completely forgotten that bit of the book. Could you do us a favour and post the table 41.5 - my regs book is in the office and I won't be there until wednesday earliest, so would love to see what it says.

 

[msb2003]

if you need to RCd circuit to meet Zs, I think I would want to be doing something about the circuit. LOL. Had completely forgotten that bit of the book. Could you do us a favour and post the table 41.5 - my regs book is in the office and I won't be there until wednesday earliest, so would love to see what it says.

Forum won`t let me insert the image!!!

When you think about it, it is only what we do with TT circuits, as we cannot achieve the Zs values due to the high Ze path of the Stake and general mass of earth.

Values are calculated to ensure touch voltage does not exceed 50V eg 50V / 30mA = 1667 Ohms, hope this helps

 

[alextrician]

Forum won`t let me insert the image!!!

When you think about it, it is only what we do with TT circuits, as we cannot achieve the Zs values due to the high Ze path of the Stake and general mass of earth.

Values are calculated to ensure touch voltage does not exceed 50V eg 50V / 30mA = 1667 Ohms, hope this helps

 

[msb2003]

Thanks Canoeboy, why couldn`t I do that !!?? ; -)