ban: you say 'Looks like TN-S, Not TT anyway, which is the only supply type which affects the CU.' Which one of these two systems (TN-S or TT) does not affect the CU please? There are no other extraneous metal parts at all in the vicinity of the two earths clamped to the cable, which might explain the status quo, the only other simple option being the thick pipe itself?
Protective device type, for supply system
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TN-S. With a TT supply you can't use a "normal" split-load board (i.e. the ones sold in the sheds-for-the-hard-of-thinking) - there are special requirements.biblio said:
It doesn't matter how far away they are - incoming service pipes must be bonded to the main earth terminal:
ban: assuming I have a serviceable TN-S supply, then in regard to a new CU can I avoid having 'cbType2' protective devices? The OSG, page 42, Table 7.1, relates these to 58 and 46metres of maximum cable length. And if these figures are based on disconnection times only, could they actually be shorter than this by applying voltage drop? I just need to know that 60metres circuit length will be okay finally with a new CU. I don't want to find that my circuit is too long after the CU is replaced or fitted with an arbitrary choice of fuse type. Voltage drop varies according to 30 or 32amp rating too; can I choose 30amp protection instead of 32amp, with a new CU, for the reason of complying with the 9v limit?
Mother Earth: my gas mains is a plastics pipe, and the water supply is a plastics pipe, where entering the building anyway. Uhm...
Mother Earth: my gas mains is a plastics pipe, and the water supply is a plastics pipe, where entering the building anyway. Uhm...
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You won't find a new din-rail mcb in 30A, I don't think.
The supply pipe coming in is not the issue with regard to bonding - it is what is on the consumer side: after the meter or stopcock.
If these are copper/metal, they will need bonding, basically as close as poss. to the meter or cock.
The supply pipe coming in is not the issue with regard to bonding - it is what is on the consumer side: after the meter or stopcock.
If these are copper/metal, they will need bonding, basically as close as poss. to the meter or cock.
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Here we go again....securespark said:
Why?
Changing a 32A mcb to a 30A does not necessarily affect voltage drop calculations, or should not. Voltage drop is calculated from the current you expect the circuit to be carrying. Arguably, if you are expecting a ring to be loaded to its full capacity, then you should be thinking about splitting it into two rings.
Damocles: my gas and water supply pipes are both over 30ft away from the electric supply (one of the two earths might well be the same earth clamped next to the gas meter though - being routed across the ceiling void of three rooms) but both gas and water mains are plastics pipe before they leave the house. They are copper pipe only up to where they both leave the house. Can I drive an appropriate rod into the soil outside to improve things eg?
If voltage drop can be assumed on the probable current used, then this makes a huge difference to circuit length doesn't it. I'd like to know what ban would advise. Would I total all the amps of what appliances might be expected to be used at the same time eg? How is drop tested by any authority anyway? The OSG uses the rating of the protective device doesn't it?
If voltage drop can be assumed on the probable current used, then this makes a huge difference to circuit length doesn't it. I'd like to know what ban would advise. Would I total all the amps of what appliances might be expected to be used at the same time eg? How is drop tested by any authority anyway? The OSG uses the rating of the protective device doesn't it?
No.biblio said:
You can only calculate voltage drop on the basis of the design current, and with a ring that is impossible to determine. If you use the worst case of 3 sockets right next to each other in the middle of the ring drawing a total of 32A, then 63m would be the maximum length of the ring. In reality, with rings, several small loads could be present at different places, and voltage drop is not the issue, it's disconnection times that limit the length of the ring, hence the different limits according to the type of protective device and the value of Ze.
It's not.
Not sure.
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Ban
What do you mean, why??
And which bit are you applying "why?" to?
Are you saying DON'T bond?
What do you mean, why??
And which bit are you applying "why?" to?
Are you saying DON'T bond?
Bonding of gas and water supplies is not done to improve the electrical earth. It may do this, but it is really the other way round. It is done to make sure the supplies are at the same voltage as the electrical earth system. Connecting an earth rod directly to the water supply where it enters the house would actually make matters worse.
The electrical earth should have its own dedicated method of connecting it to the outside world earth. Here this seems to be by connecting it to the cable sheath coming in. That is how it is intended to work, and should be fine. There is one point alongside where your electrical supply comes in which is the official reference earth for the whole house.
The argument above is about whether the connections to the sheath are good enough. It is not just a question of resistance across the joint, but also current carrying capacity if there was a fault. The middle of a big short circuit would not be a good time to discover that the lead clamp was the weak point acting as fuse.
If you look at the footnotes to table 7.1 in the OSG, you will see it states cable lengths are based upon voltage drop unless otherwise marked. For the rings listed, it is not immediately obvious what the lengths are based on as they mostly have different footnotes. I think the table is calculated to give you worst-case safe lengths, without having to do explicit calculations of things which might allow you to have a longer cable.
A type 2 mcb is a now obsolete standard about how much current is needed to operate the breaker. You would now expect to use a type B, which gives the much happier length of 88m rather than 58m for 30A ring. Possibly you would use a type C for lighting, but not otherwise.
The question of whether you have or need an rcd does not affect your choice of a type B. Any short circuit between neutral and live will operate the mcb. A short between earth and live will operate the mcb provided the earth is good enough. The question of what supply youy have and how good an earth it provides applies here. If the earth is not good enough, then you have to use an RCD. You may have an RCD anyway, even where it is not absolutely required, because it should operate faster and trap very small shorts to earth.
The electrical earth should have its own dedicated method of connecting it to the outside world earth. Here this seems to be by connecting it to the cable sheath coming in. That is how it is intended to work, and should be fine. There is one point alongside where your electrical supply comes in which is the official reference earth for the whole house.
The argument above is about whether the connections to the sheath are good enough. It is not just a question of resistance across the joint, but also current carrying capacity if there was a fault. The middle of a big short circuit would not be a good time to discover that the lead clamp was the weak point acting as fuse.
If you look at the footnotes to table 7.1 in the OSG, you will see it states cable lengths are based upon voltage drop unless otherwise marked. For the rings listed, it is not immediately obvious what the lengths are based on as they mostly have different footnotes. I think the table is calculated to give you worst-case safe lengths, without having to do explicit calculations of things which might allow you to have a longer cable.
A type 2 mcb is a now obsolete standard about how much current is needed to operate the breaker. You would now expect to use a type B, which gives the much happier length of 88m rather than 58m for 30A ring. Possibly you would use a type C for lighting, but not otherwise.
The question of whether you have or need an rcd does not affect your choice of a type B. Any short circuit between neutral and live will operate the mcb. A short between earth and live will operate the mcb provided the earth is good enough. The question of what supply youy have and how good an earth it provides applies here. If the earth is not good enough, then you have to use an RCD. You may have an RCD anyway, even where it is not absolutely required, because it should operate faster and trap very small shorts to earth.
Thanks indeed for your replies Damocles, ban-all-sheds, securespark.
ban wrote:
'it's disconnection times that limit the length of the ring, hence the different limits according to the type of protective device and the value of Ze.'
Is Ze the earth loop impedance? Can this only be measured by my REC? Is it a factor that can be improved easily enough, if necessary? Is it related to Main Equipotential Bonding perhaps?
Damocles wrote;
'It is done to make sure the supplies are at the same voltage as the electrical earth system'. (Thanks, biblio now fully understand meaning of equi joined to potential. Like all guests coming into my house put on same carpet slippers, not when go outside house with wellies clogs sandals socks on, yes?)
'type 2 mcb' (obsolete breaker current); is this the OSG's 'cb Type 2'?
Does Type B include all/any RCD, or is RCD a separate type? (How generic are these type names?)
ban wrote:
'it's disconnection times that limit the length of the ring, hence the different limits according to the type of protective device and the value of Ze.'
Is Ze the earth loop impedance? Can this only be measured by my REC? Is it a factor that can be improved easily enough, if necessary? Is it related to Main Equipotential Bonding perhaps?
Damocles wrote;
'It is done to make sure the supplies are at the same voltage as the electrical earth system'. (Thanks, biblio now fully understand meaning of equi joined to potential. Like all guests coming into my house put on same carpet slippers, not when go outside house with wellies clogs sandals socks on, yes?)
'type 2 mcb' (obsolete breaker current); is this the OSG's 'cb Type 2'?
Does Type B include all/any RCD, or is RCD a separate type? (How generic are these type names?)
Yes, it's the earth loop impedance extrernal to your installation, normally measured at the incoming tails to your main isolator.biblio said:
No, any electrican with the right equipment cam measure it.biblio said:
Not by you. As stated above it's external to the installation and not your responsibility.biblio said:
No. In fact to measure it you have to disconnect the main equipotential bonding - which is there, as stated previously to make sure that all means of bringing an earth potential into the property are at the same earth potential.biblio said:
Yes. BS 3871 covers the older circuit breakers whose types are numbered. This has been superseded by BS EN 60898 (Type B/C/D) but figures are included in the OSG for types 1, 2 & 3 because many installations still use them.biblio said:
biblio said:
Miniature Circuit Breakers (MCB) to BS3871/60898 are combined overcurrent and short-circuit protective devices. The type (B, C, D) describes their operating characteristics and in almost every domestic situation Type B is the appropriate one. Residual Current Devices (RCD) are a different type of device and detect current imbalance between phase and neutral, i.e. leakage of current from the circuit, for example when you stick your fingers into an opened socket and provide an alternative earth path. Modern RCD's are described by BS EN 61008 & 61009
Hope that helps (and sorry to butt in... I just happened to be passing)
dingbat: thanks for passing by.
Damocles wrote:
'The question of what supply you have and how good an earth it provides applies here. If the earth is not good enough, then you have to use an RCD.'
Where I asked about RCD (with Type B), I was meaning to say RCBO (though I am not confident if RCBO is the CU 'equivalent') not the portable plug-in RCD product.
Put another way, is 'cb/RCBO Type B' (Table 7.1 in the OSG) the same as Type B?
Damocles wrote:
'The question of what supply you have and how good an earth it provides applies here. If the earth is not good enough, then you have to use an RCD.'
Where I asked about RCD (with Type B), I was meaning to say RCBO (though I am not confident if RCBO is the CU 'equivalent') not the portable plug-in RCD product.
Put another way, is 'cb/RCBO Type B' (Table 7.1 in the OSG) the same as Type B?
It's the usual disagreement.securespark said:
If a pipe entering the building is plastic, then it is not providing a path to earth, and therefore nothing connected to it can provide a path to earth, so there is no need to bond subsequent metal pipes to ensure that they are at the same potential as the installation earth....
biblio said:
Yes. RCBO = Residual [current device] Circuit] Breaker with Overcurrent protection. (There are variations on the exact wording) It's a combined RCD and MCB and a Type B has the overcurrent characteristics of type B circuit breakers.
RCD's come in various guises - plug-in, for protection for individual appliances, two-module types which protect one or more circuits in your installation - mounted in your CU or as additions - or RCBO's providing both overcurrent and supplementary protection for individual circuits)
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