TT system

[malcolmsanford]

Everything seems to be covered regarding the earthing on here now and regardless of what earth system is at the origin I like wirepuller would use that earth to protect the SWA by connecting the armour at that DB and then leave it open at the new 3 phase DB.

Again as wire says I you would batch the connections points to the maximum allowed which is 4 and rod each point of connection, so 1 rod would do your 4 sockets. Though of course each Ra would be taken at each point until IMO you got below 100ohms, I know it is 200 in the BS 7671-2008 but I would work to BS 7430 in this case.

The only thing I would bring to the table is perhaps running the points in a ring main. As 300 mts being your furthest point by doing in a ring main you could utilise a smaller SWA

 

[sparks1234]

the incomer is a tn-s

 

[Deleted member 9648]

the incomer is a tn-s

You do not need to separate the earth at all then,you can use it for the entire system as long as there is no possibility it will ever be converted to TNCS....your call on that one.
708.411.4

 

[yellowvanman]

Wouldn't making a 300+m ring main be a lot more expensive? Even if you half the cable size, twice the amount of smaller cable will be more expensive. Could be the difference between getting the job or not.

 

[malcolmsanford]

It will depend on your cable calcs, you have to factor in loading, volt drop and erection methods as in all calculations, and weigh up wether say a 70mm ran out in a radial will be more expensive than say maybe a 25mm in a ring. Remember the smaller the CSA the easier to work with installation, connections, type of accessories used etc etc, all this should be as factor in your design.

After all most large installations are done in a ring main configuration, it may be the case that for this installation a radial will be better, but a ring main is another option.

 

[sparks1234]

well, our thought is that the radial is rated at say 40 amps, where we have the sockets we were planning on a resin joint with a 6mm going to the post. not really sure why you guys are talking about 4 sockets per stake, the posts would be 50 mtrs apart so imo seperate stake makes sense

 

[malcolmsanford]

Reg 708.530.3 in the note advises that no more than 4 socket oulets in a group in a single location. So if you had 4 caravans, tent pitches or camper vans , and there are 4 of them witin 20 metres of a supply then they can all be fixed to a single position, but that is advised as a maximum

 

[Engineer54]

This is not correct....reg 708.553.1.14 only states that the CPC of each socket outlet shall be connected to an earth electrode complying with the requirements for a TT...not that each socket shall be connected to a separate earth electrode....this technically only applies where the supply is TNCS as well,a TNS supply can be used for the entire install without the need to separate and TT anything,although IMO it would be preferable to TT beyond the DB regardless of the source earthing system.

The usual provision is power boxes incorporating the fault protection devices at the caravan pitches, each serving no more than 4 sockets/pitches. 4 separate rods at one location would not be practical or necessary....there would of course be advantages to providing one rod at each power box location rather than relying solely on a single rod at the DB end....and yes DP RCBO's would be fine but not single pole.

If you read my post in totality, you would have seen that it depended on what type of caravan connection unit was being employed, i quoted a 2 way and you and others have quoted a 4 way unit!!! obviously then each ''unit would require a rod , rather than each pitch location. ...Oh and TNC-S (PME) is not permitted to be the supply means to caravan installations!!

Now that we know it's a TN-S earthing system supplying this installation, there is little point in roding anything, unless you want to future proof the installation against a TNC-S supply. You certainly won't need to separate the SWA or anything else, on the earthing side of things presently.

There are several ways to supply these caravans, a ring as has been suggested or a parallel radial for argument's sake, both using much smaller sized cables than stated by the OP. Or, keep the original idea of a central DB and run 2 or 3 distribution rings/parallels to feed the caravan pitches/connection units. I wouldn't use joints to tee off the distribution cables, much better all round to have access to all such connections at the pitch connection units themselves.

As been stated, you need to do your calculations to find the right sized cable to use on any such system(s) you decide to employ on this installation. I would be calculating on several different installation methods, along with the costs associated with each method, to find the most suitable. I'm not sure, but i think you will also need some form of site lighting system, especially for any laid out pitch access roads, be they tarmac or gravel type roads, ...need to check minimum lux level requirements!! All adds to the cost and running costs of the installation, ...lol!!

What load are you allowing for each caravan?? I wouldn't go higher than 10A max!! especially if this is a holiday venue site. (big difference if it's a resident mobile homes type site..lol!!) There are websites, that would be able to give reliable information on this and other aspects of planning such caravan site...

Any chance of posting a plan/sketch layout of this proposed caravan site?? Makes things easier to visualise and comment on....

 

[Deleted member 9648]

If you read my post in totality, you would have seen that it depended on what type of caravan connection unit was being employed, i quoted a 2 way and you and others have quoted a 4 way unit!!! obviously then each ''unit would require a rod , rather than each pitch location. ...Oh and TNC-S (PME) is not permitted to be the supply means to caravan installations!! I know..if you read my posts you will see that was stated and that the relevant regulations were quoted....my post clearly states that a TNCS must be converted to a TT and that a TNS can be used,there is in fact no requirement for a separate rod at each station if the supply earth is to be separated,one rod at the point of separation would meet the requirements.....interesting that you now seem to be giving the information I have already posted....please refer to the entire thread and section 7 of the BRB before jumping in

Now that we know it's a TN-S earthing system supplying this installation, there is little point in roding anything, unless you want to future proof the installation against a TNC-S supply. You certainly won't need to separate the SWA or anything else, on the earthingside of things presently.

There are several ways to supply these caravans, a ring as has been suggested or a parallel radial for argument's sake, both using much smaller sized cables than stated by the OP. Or, keep the original idea of a central DB and run 2 or 3 distribution rings/parallels to feed the caravan pitches/connection units. I wouldn't use joints to tee off the distribution cables, much better all round to have access to all such connections at the pitch connection units themselves.

As been stated, you need to do your calculations to find the right sized cable to use on any such system(s) you decide to employ on this installation. I would be calculating on several different installation methods, along with the costs associated with each method, to find the most suitable. I'm not sure, but i think you will also need some form of site lighting system, especially for any laid out pitch access roads, be they tarmac or gravel type roads, ...need to check minimum lux level requirements!! All adds to the cost and running costs of the installation, ...lol!!

What load are you allowing for each caravan?? I wouldn't go higher than 10A max!! especially if this is a holiday venue site. (big difference if it's a resident mobile homes type site..lol!!) There are websites, that would be able to give reliable information on this and other aspects of planning such caravan site...

Any chance of posting a plan/sketch layout of this proposed caravan site?? Makes things easier to visualise and comment on....

.............

 

[Engineer54]

Are you seriously stating that a single rod at that central DB, is sufficient for all those caravan connection stations ..... hahaha!!!! In the real world a rod IS required at each pitch or station, ....and all preferably linked to bring the Ra to a functional level....

You would actually totally rely on an RCD device knowing of there failure rates, rather than installing a TT system that ''works'' on a installation such as this one???


What information are you talking about?? I'm only relating to what i have previously posted and what others have subsequently posted on this subject...

 

[sparks1234]

Guys
In this particular case the pitches will be strung out over a couple of hundred meters in basically a straight line and each will be about 35 mtrs apart. Each post has 2 outlets, each with seperate RCD installed. It will be so simple to come out of the post and bang a stake in.

 

[Deleted member 9648]

Are you seriously stating that a single rod at that central DB, is sufficient for all those caravan connection stations ..... hahaha!!!! In the real world a rod IS required at each pitch or station, ....and all preferably linked to bring the Ra to a functional level....

You would actually totally rely on an RCD device knowing of there failure rates, rather than installing a TT system that ''works'' on a installation such as this one???


What information are you talking about?? I'm only relating to what i have previously posted and what others have subsequently posted on this subject...

You need to refer to section 7 of bs 7671. All the info I have given is in accordance with the requirements of bs 7671 for caravan site installations and I have quoted the reg numbers in support of that info.
Unfortunately you seem to make up your own regulatons and think you know better than bs 7671.

Regarding the highlighted section of your post, FYI in the real world a TT system relies on an RCD for fault protection....one rod at the DB can serve that RCD and any number of RCD's upstream of it ....I did not say that is how I would do it,but that it would comply with the regulations.If you read all my posts you will see that I have already stated that there would be advantages to one rod at each station.BUT IT IS NOT A REQUIREMENT OF BS 7671...I would also point out that on a TT it is not permitted to rely on a low Ra "to a functional level" for protection of socket outlets...an RCD is required,regardless of their "failure rates".As previously stated you seem to ignore bs 7671 and invent your own version.

 

[Engineer54]

And that my friend is why on occasions i do not take notice of BS7176!!! It's not a bible, it's a guidance and relates to ''Minimum'' requirements...

I don't invent or make up my own rules, it's pure electrical theory. Do you honestly think Design Engineers rely on BS7176 when designing a functional TT system?? High Ra values isn't going to protect against anything!! In fact no need for a TT system using your analogy, as the RCD is there to protect against earth faults right?? ...lol!! Except that is when it, or they Fail...

I don't need teaching about TT systems, i've been involved in installing far more than most here, from vast ground fields, power station ground fields, IT ground fields, lightning protection systems, project wide linked systems, to stand alone building TT systems. I know the few advantages of a TT system, to the numerous disadvantages, ...especially when they aren't capable of tripping a protective device, (which is why i keep stating here, that anything over a 20 ohms Ra isn't going to be doing too much....)

So lets see, ..what would you install?? that single rod you talk about, or the rod at each station or pitch?? Would you link those rods, or would you leave them as stand alone rods??

 

[Deleted member 9648]

And that my friend is why on occasions i do not take notice of BS7176!!! It's not a bible, it's a guidance and relates to ''Minimum'' requirements...

I don't invent or make up my own rules, it's pure electrical theory. Do you honestly think Design Engineers rely on BS7176 when designing a functional TT system?? High Ra values isn't going to protect against anything!! In fact no need for a TT system using your analogy, as the RCD is there to protect against earth faults right?? ...lol!! Except that is when it, or they Fail...

I don't need teaching about TT systems, i've been involved in installing far more than most here, from vast ground fields, power station ground fields, IT ground fields, lightning protection systems, project wide linked systems, to stand alone building TT systems. I know the few advantages of a TT system, to the numerous disadvantages, ...especially when they aren't capable of tripping a protective device, (which is why i keep stating here, that anything over a 20 ohms Ra isn't going to be doing too much....)

So lets see, ..what would you install?? that single rod you talk about, or the rod at each station or pitch?? Would you link those rods, or would you leave them as stand alone rods??

You cant even give it the correct Bs number....do you actually have a copy to refer to?

As regards the final part of your post any of the rod arrangements would meet the requirements (411.5.1)..as long as the final measured Ra was <200 ohms.....you seem to be missing the point that on this install multiple RCd's will be used.There will be a time delayed 100ma rcd at either the intake or DB..and each and every socket will have a dedicated 30ma rcd...on top of that a caravan plugged into the socket will have an rcd protecting it's internal installation. So I make that three rcd's that will have to fail before the end user of a caravan is put in real danger by an Ra of <200 ohms.
You say you have installed many TT systems and seem to think that an Ra low enough to operate an overcurrent device is essential....please tell me how you would achieve that low Ra on a small domestic?....10 or 12 rods banged in all over the garden?

 

[Engineer54]

Trying to be clever over a typo?? lol!! You stick to your 200 ohms Ra, your obviously a one, or single rod man.... good luck to you!!!

For your information, i am on a TT system here!! I have brought Ra values down to below 20 ohms by the use of just 2 further rods to the system, on quite a few of my friends properties here. Mine is 3 ohms in winter and 5 to 8 ohms in the summer, but then i made the provision of having the steelwork of my house foundation to be available for connecting to my TT system, along with other structures steelwork on my property!! There is a hell of a lot more to TT systems than bunging a rod in the ground!!

Once you have a decent Ra value, you then have to maintain that value, ...Now how many electricians in the UK that quite happily create a so-called TT system, can instruct the home owner or client how to maintain those systems, ...not many at all, i fear!!!

Your right, i don't have the current BS7671 as yet, Does that mean i don't know what i'm talking about?? On this subject, i think i'll rely on my training, my working experience, and the knowledge i've gained over the 35 years i've been in this industry, ...after all, my employers are more than happy too!!!