lecsoft weird question

[rustynails]

on a lecsoft simulator I came across this...

When working on a construction site, the result of the resistance of the earth electrode and the rated residual operating current should not exceed

a)110v
b)50v
c)25v
d)230v

i put 110v not really understanding the question... but the answer was 25v confused.com

 

[telectrix]

50V touch voltage is the answer. that's why the max. Zs for an RCD protected circuit is 1667. ( 1667 x 0.03 = 50V ).

 

[Baker1988]

I would of said 50V also but if the answer is 25V then im also confused

 

[shanky887614]

nicked this from iet

--------

Construction sites are governed by section 604 of BS7671.

The normal 7671 regs apply inside the site hut and you can use the TNCS (PME) supply. It is a good idea anyway to protect the installation with a 30mA RCD.

The 4 double sockets you mention " on a site board". Hopefully this is not outside as this is not permitted. Portable tools have to be plugged in to 110V centre tapped transformers.


You cannot take the PME supply outside so you will have to TT it separating the supply earth and putting in an earth electrode.

The earth loop impedance values for TN systems are listed in in 604 for various voltages for 0.2s. These only apply to TN systems. If you are using an RCD these do not apply. The maximum touch voltage is reduced to 25V so the resitance of the earth electrode (Ra) mutilpied by the operating current of the RCD must be equal to or less than 25V. The 110V loop impedances can be tested with some loop impedance testers if they will go down to 110V.

If the REC will not supply without a certificate then you will have to limit the test to " no supply available dead testing only further testing required when supply is connected". I have done a number of these to get a supply re-connected to houses on change of occupancy. Re test when the supply is connected. Use the normal certificates.

The interval for testing construction sites is 3 months.

Does this help?

Regards

John Peckham

-------------------------
John Peckham

 

[Baker1988]

have a look at swindon massive it isis better as it explains why so you can research it yourself.

because of how the 110v transformers work.

50v phase to earth
110v phase to phase

I totally agree with ya and i understand about 110V transformers 50V to earth but why has the op stated the answer was 25V thats not the correct answer is it

 

[Baker1988]

Deleted

 

[shanky887614]

I totally agree with ya and i understand about 110V transformers 50V to earth but why has the op stated the answer was 25V thats not the correct answer is it

the question wasnt asked very well i bet it was touch voltage after a quick google

 

[Baker1988]

the question wasnt asked very well i bet it was touch voltage after a quick google

Yeah it does make sence now lol

 

[telectrix]

seem to have in back of brain that max. touch volts on building site used to be 25V, but is now 50V.

 

[shanky887614]

seem to have in back of brain that max. touch volts on building site used to be 25V, but is now 50V.

thats because the example i chose was TN and you chose tt

the max touch voltage is ohms / disconnection time

for tt thats 1667 ohms allowed for electrode resistance

50 / 0.03 = 1667

1667 / 30ma = 50v touch voltage

ohms / rcd rating = touch voltage

i think thats where they are getting the touch voltage from

feel free to correct/ tell me im an idiot but this is what ive just researched

 

[telectrix]

where does the 0.75 come from?

and you have too many zeros knocking about. 30mA is 0.03, not 0,003

 

[shanky887614]

where does the 0.75 come from?

and you have too many zeros knocking about. 30mA is 0.03, not 0,003

no idea, as its 0.8 for tns.

yeah im using phone for calcs and posting on here with the same sorry

 

[HandySparks]

thats because the example i chose was TN and you chose tt

the max touch voltage is ohms / disconnection time

for tt thats 1667 ohms allowed for electrode resistance

50 / 0.03 = 1667

1667 / 30ma = 50v touch voltage

ohms / rcd rating = touch voltage

i think thats where they are getting the touch voltage from

feel free to correct/ tell me im an idiot but this is what ive just researched

You may or may not be an idiot, but you're certainly confused.

"the max touch voltage is ohms / disconnection time"

Disconnection time isn't relevant here; I'm guessing you really mean rated residual operating current.

Now go back to basics: V=IxR, not R/I

 

[shanky887614]

You may or may not be an idiot, but you're certainly confused.

"the max touch voltage is ohms / disconnection time"

Disconnection time isn't relevant here; I'm guessing you really mean rated residual operating current.

Now go back to basics: V=IxR, not R/I

 

[HandySparks]

...So?

 

[shanky887614]

...So?

read the op

 

[Engineer54]

Now come on, this is all really BASIC stuff were talking about here!!!

 

[HandySparks]

Now come on, this is all really BASIC stuff were talking about here!!!

Is that aimed at me, E54?

 

[Engineer54]

NO, ...It's directed towards the thread itself, ....Safe touch voltage requirements and the calculations to obtain touch voltage values is very basic stuff!!

 

[Silly Sausage]

nicked this from iet

--------

Construction sites are governed by section 604 of BS7671.

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

I think the OP's question is well out of date!

Shanks, you need to ensure your research is upto date, 604 is the obvious giveaway! :smilielol5:


The 25V and 0.2s were dropped with the introduction of the 17th (see page 6 BGB), so the answer is 50V.

 

[telectrix]

cheers archie for confirming what i said in post#2. apart from eng54's post and 1 or 2 others, several posts have been absolute bollox.