Generator

[Andy™]

so why does the genny require a rod then?why do the DNO rod star point on a TNS distribution?

to make the install a TT is explained in my post,

you could make the installation TT, but you would then need 2 rods min, one at star, and one at installation (and you would have to ditch earth connection between the 2). but if the genny gives TNS, why bother with TT? just creates extra work, and extra risk, for no reason.

the whole reason it has a rod is so there is a return path to star, so if someone was to touch a live conductor, it can flow through earth, back to star, and trip any RCD. no rod at star means under fault conditions (to ground/soil) will mean a voltage between genny frame/earth, to ground.

unless you omit the earth between genny & installation, and use a rod at each end, the supply will be TNS

i would draw you a diagram to illustrate this, but since TNS is well known, there is already a diagram in 7671 (fig 2.3, page 33, and then fig 2.5, page 34)

you welder theory, whilst i agree with the results of the welder not working with a small genny, has absolutely nothing to do with earthing arrangements

 

[steptoe]

[quote name='Andy

 

[Andy™]

you really make no sense....

go look at the diagrams of earthing in 7671 - you will see that unless you use 2 rods and ditch earth between, its TNS. there is absolutely no need to make it TT. there never was a TNCS part to it, so there is no safety issue with 'exporting' the earth

the point about 'will mean a voltage between genny frame/earth, to ground.'

if you have a high resistance (i.e genny sitting on tarmac, no rod), then if there was a fault to ground, there would be a large voltage between ground and earth. if you add a rod, current will be able to flow from the ground to earth (/star) much easier, and there will be much less voltage between ground & earth

 

[steptoe]

[quote name='Andy

 

[Andy™]

just read it,its the same principle ,

you have 2 sources of earth sitting at two different potentials,

headbangheadbangheadbang

late now, good night

you dont have 2 sources of earth. you have 1. you have a TNS where earth for installation comes directly from star point. the ground has a rod to take any current that does go to ground directly to star. its the exact same as DNO having a rod at the star point of transformer, and giving a TNS supply.

try drawing a diagram (sicne those in 7671 are obviously wrong) and show exactly where you have 2 different earth sources. its something i would love to see. or go to fig 2.3 (as i mentioned before) and point out exactl where this 'second' earth thats at a different potential is

 

[batty]

I would wait and see it may need rodding it may not depending on type of gennie it is.

 

[Watchdog]

Andy

 

[M107]

DB having just (3 months ago) been on a Fischer Panda course, we were advised/instructed to rod the genny at the genny end.

Who am I to argue, I know sod all about gensets :coat .... maybe a little more than I did.

 

[binky]

DB having just (3 months ago) been on a Fischer Panda course, we were advised/instructed to rod the genny at the genny end.Who am I to argue, I know sod all about gensets :coat .... maybe a little more than I did.

Cocurr, TT TN-s doesn't really matter, following the manufacturers instructions is best option either way. do this ut the kettle on and read carefully O)

 

[AndyD1]

Mr B,

Chapter 55 of BS7671 is worth a quick scan and may answer your original query.

 

[steptoe]

If you did indeed tt the installation you would more than likely render the Fault protection provided by the fuses and mcbs useless.The only rod being installed would be one made for your own back ROTFWL

This would then require an rcd or many, as discrimination now becomes an issue.

Best to check that the genny can preform under fault conditions so that fault protection is provided by the overload protective devices.

This would require the installation to be installed in essence as a tn-s as already mentioned in previous posts.

Regards chris

how do you reckon to that?

once you use a genny as a supply then your whole installation must now comply with 17th as you have changed the whole installation, so RCDs would be required anyway.

as for how we have two earths,

you have the artificial earth provided by star point(genny or otherwise) and the actual mass of earth that we stand on and that is what conducts the fault current, hence the necessity to rod all supplies,

and once the fault flows through you under fault conditions to actual earth then ALL installations become TT.

do I need to do a drawing?

 

[Andy™]

do I need to do a drawing?

yes

 

[green-hornet]

[quote name='Andy

 

[Lee321]

pop corn

 

[steptoe]

Steptoeas for how we have two earths,

you have the artificial earth provided by star point(genny or otherwise) and the actual mass of earth that we stand on and that is what conducts the fault current, hence the necessity to rod all supplies,

and once the fault flows through you under fault conditions to actual earth then ALL installations become TT.

do I need to do a drawing?

So am i right in saying your earth loop is as follows:

transformer winding/genny winding, line conductor, exposed conductive part,cpc, Barefooted volunteer to touch exposed conductive part whilst sticking feet in the general mass of earth,General mass of earth, source electrode and finally the windings again.

So lets hope enough current flows through the volunteer to disconnect supply

Hope he is paid well too

Or is the volunteer you

Regards chris

close,

except without the cpc, and the volunteer doesnt need to be barefooted.

how do you suppose direct contact disconnects?

now suppose this happens with NO RCDs fitted, as not all installs comply with the current regs.

and you have no star point to earth continuity, because you have no rod at star point,.

this is why all star points must be earthed.

I will do a pic on my phone and upload it,

scanners etc seem to be beyond me.

 

[Sidewinder]

steps,

I know the technology problem & I used to program cnc's, plc's, servo's and inverters!

 

[Lee321]

steps,I know the technology problem & I used to program cnc's, plc's, servo's and inverters!

ah come on programming an inverter thats playing up the role :^O

you set the settings i think you'd call it

 

[steptoe]

as the person hasnt actually touched an extraneous part that has become live under fault conditions then please explain how else the current returns to star.

thanks.

 

[Sidewinder]

ah come on programming an inverter thats playing up the role :^O you set the settings i think you'd call it

Not when you work for the inverter manufacturer and you have to develop the settings for the machine OEM!

 

[Sidewinder]

steps, that is direct contact is it not?