17th Edition (C&G 2382) Couple of questions, help

[roys]

Hi Folks

In the middle of doing my 17th Edition course just now. Couple of things I have taken a mind blank on, can any one help please?

Table 41.5 on page 50 BRB, How come the earth fault impedance Zs is lower in the 230 V to 400V column yet it is a higher Zs in the lower and higher voltage columns?

Examples of SELV and PELV? I am thinking of some bathroom extractor fans and ELV down lighters for SELV, but I am struggling to think of PELV stuff, any ideas?

I may be back through the week with more questions.

Thanks in advance Steve

 

[sparkytim]

Hi Steve

Not sure on first question, as only working with Uo(Voltage to EARTH) of 230 Volts have not thourght about it, but Iam sure someone will be along with the answer.

As to SELV, PELV AND FELV, my understanding is;

SELV SEPERATED from earth ELV, i.e. doorbell system

PELV PROTECTIVE ELV NOT seperated from earth i.e. FIRE ALARM SYSTEM

24 volt SYSTEM Earth used

FELV FUNCTIONAL ELV NOT used for safety purpose but just FUNCTIONAL requirement for equipment using it.

We will see if any one has any other ideas

 

[Apache]

FeLV is the common abbreviation for Feline Leukaemia Virus - unlikely to be much help here...

 

[roys]

FeLV is the common abbreviation for Feline Leukaemia Virus - unlikely to be much help here...

Thanks patch I will mention that on the course tomorrow Think I also prefer your version.

 

[roys]

OK bit of a bump for yesterdays questions and a new one:

In the middle of doing my 17th Edition course just now. Couple of things I have taken a mind blank on, can any one help please?

Table 41.5 on page 50 BRB, How come the earth fault impedance Zs is lower in the 230 V to 400V column yet it is a higher Zs in the lower and higher voltage columns?

Examples of SELV and PELV? I am thinking of some bathroom extractor fans and ELV down lighters for SELV, but I am struggling to think of PELV stuff, any ideas?

OK todays new question:

Reg 521.5.2 states that Single core aroured cable shall not be used for an AC supply. Don't understand that one as we do it all the time if we are getting up to the bigger size cables, for example we may use 3 single core 300mm2 SWA cables to feed a decent sized dist board, if we were to use a 3 core cable of that size it would have a huge. overall diameter. We do gland it onto a non ferrous plate. So how can this reg be true?

Cheers Steve

 

[AAAMUSEMENTS]

OK todays new question:Reg 521.5.2 states that Single core aroured cable shall not be used for an AC supply. Don't understand that one as we do it all the time if we are getting up to the bigger size cables, for example we may use 3 single core 300mm2 SWA cables to feed a decent sized dist board, if we were to use a 3 core cable of that size it would have a huge. overall diameter. We do gland it onto a non ferrous plate. So how can this reg be true?

Cheers Steve

Because the armour should always be earthed, I assume.

If you were to use single core then the armour would have to act as the neutral?

(Insert usual I-am-not-qualified-disclaimer here...)

 

[Zeespark]

You would generally use a 4 core/3 line cable for a 300 amp incomer, 3 lines + neutral.

AndyGuinness

 

[Risteard]

All circuit conductors should be contained within the same ferrous enclosure. This would not be the case with a single core SWA cable.

 

[roys]

Sorry Risteard, I am not sure what you mean, the single core cables leave for example the generator term box glanded via a non ferrous gland plate and then enter a switchboard again glanded onto a non ferrous gland plate. The main switchboard and generator enclosures are themselves ferrous.

The question I am asking is that we regulary use single core SWA cables for our bigger size loads, the cables are run as a set, but the are defiently single core SWA cables yet the BRB reg 521.5.2 appears to say I can't do it, or am I reading it wrong.

Cheers Steve

 

[AAAMUSEMENTS]

Sorry Risteard, I am not sure what you mean, the single core cables leave for example the generator term box glanded via a non ferrous gland plate and then enter a switchboard again glanded onto a non ferrous gland plate. The main switchboard and generator enclosures are themselves ferrous.The question I am asking is that we regulary use single core SWA cables for our bigger size loads, the cables are run as a set, but the are defiently single core SWA cables yet the BRB reg 521.5.2 appears to say I can't do it, or am I reading it wrong.

Cheers Steve

If the core is single and carries the phase, then what carries the neutral?

The armour can't as it should be earthed...

:|

 

[sparkytim]

Hi again

The only thing I think it may mean is you can not use a SINGLE core SWA ON ITS OWN for an AC circuit i.e. Core for L and armour for N.

(DNOs can as supply CNE cable BUT WE CAN NOT)

When used as L Phase ONLY and armour earthed along with 3 OR 4 OTHERS

then that seems ok.

Example of a PELV would be a ELV circuit with a connection to earth on the secoundary side.

 

[roys]

Hi sparkytim, Yip like that explanation, it is funny how sometimes you can read these regs and just not understand what they are trying to get at.

Hi aaamusements, I was thinking of 3 phase systems where a neutral is not always needed, of course it could just as easily be a 3 phase and N supply. Re read your first post again and I see where you arre coming from as well.

Strange how no one has come up for an answer for the Table 41.5 question yet, I thought it was me just me a bit mind melted as well.

Thanks all for input so far Cheers Steve

 

[green-hornet]

Strange how no one has come up for an answer for the Table 41.5 question yet, I thought it was me just me a bit mind melted as well.

Thanks all for input so far Cheers Steve

Its all in the calculation 411.5.3 (i) and (ii)

 

[ADS]

Hi FolksTable 41.5 on page 50 BRB, How come the earth fault impedance Zs is lower in the 230 V to 400V column yet it is a higher Zs in the lower and higher voltage columns?

Hi Steve,

It's to do with fault protection on a TT system and the maximum allowed disconnection times.

As mentioned by Green-Hornet, the two requirements in 411.5.3 need to be met when a RCD is used for fault protection - i.e. the touch voltage formula and the max disconnection time.

In the column that you are referring to, (where the Zs is lower), the disconnection time for a TT system is 0.07 seconds (Table 41.1).

Using a 30mA RCD as an example -

Although 1667 ohms satisfies the touch voltage equation:

50/0.03 = 1667

it doesn't satisfy the disconnection time:

A current of approximately 150mA is required to flow in order to disconnect in 0.07 sec (pg 243 in 'BS 7671)

Using the lower voltage in the range for that column

230/0.15 = 1533 ohms

This lower impedance figure also satisfies the touch voltage equation, so it is given as the max Zs for that voltage range for a 30 mA RCD.

The principle is the same for the other RCDs.

Hope that helps.

 

[SPECIAL LOCATION]

Sorry Risteard, I am not sure what you mean, the single core cables leave for example the generator term box glanded via a non ferrous gland plate and then enter a switchboard again glanded onto a non ferrous gland plate. The main switchboard and generator enclosures are themselves ferrous.The question I am asking is that we regulary use single core SWA cables for our bigger size loads, the cables are run as a set, but the are defiently single core SWA cables yet the BRB reg 521.5.2 appears to say I can't do it, or am I reading it wrong.Cheers Steve

Reading it wrong IMHO...

521.5.2

Is in relation to Ferromagnetic enclosures & Single SWA's

NOT

single SWA's themselves into NON-Ferromagnetic enclosures!!

Look at the bold heading bit!

Guinness

 

[AAAMUSEMENTS]

Hi aaamusements, I was thinking of 3 phase systems where a neutral is not always needed, of course it could just as easily be a 3 phase and N supply. Re read your first post again and I see where you arre coming from as well.

Got it, makes sense now!

 

[roys]

Thanks everyone for you help.

Probably have another question later on today.

Cheers Steve

 

[JUD]

Hi Steve,It's to do with fault protection on a TT system and the maximum allowed disconnection times.

As mentioned by Green-Hornet, the two requirements in 411.5.3 need to be met when a RCD is used for fault protection - i.e. the touch voltage formula and the max disconnection time.

In the column that you are referring to, (where the Zs is lower), the disconnection time for a TT system is 0.07 seconds (Table 41.1).

Using a 30mA RCD as an example -

Although 1667 ohms satisfies the touch voltage equation:

50/0.03 = 1667

it doesn't satisfy the disconnection time:

A current of approximately 150mA is required to flow in order to disconnect in 0.07 sec (pg 243 in 'BS 7671)

Where does it tell you this????

Using the lower voltage in the range for that column

230/0.15 = 1533 ohms

This lower impedance figure also satisfies the touch voltage equation, so it is given as the max Zs for that voltage range for a 30 mA RCD.

The principle is the same for the other RCDs.

Hope that helps.

Not seeing anything on page 243.

 

[ADS]

Not seeing anything on page 243.

The table at the bottom of page 243 in the 'Regs' gives you the time/current characteristics of RCDs - a bit like your time/current graphs for fuses and MCBs on the following pages.

Apology accepted, when you're ready. ; \

 

[JUD]

The table at the bottom of page 243 in the 'Regs' gives you the time/current characteristics of RCDs - a bit like your time/current graphs for fuses and MCBs on the following pages.Apology accepted, when you're ready. ; \

Sorry. I think you've taken my post the wrong way. I wasn't being pedantic.

What I was trying to ask you is where it says that a current of 150mA needs to flow to disconnect in 0.07 secs. The table on pg 243 doesn't tell you this, it just has the min. and max. disconnection times for the different devices.