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Topic Title: 200 amp three phase tt supply
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Created On: 22 July 2012 12:06 PM
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 22 July 2012 12:06 PM
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21stcentury

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Hi all, this is my first ever post on anything like this so just dipping my toes in.
Background- existing large retail unit has been spilt into 2 retail units. 1 unit is utilising the existing 400amp Three phase TN C S supply. The other unit is now being supplied by 200amp TT (sse wouldnt provide earthing connection as the have steel members are common to the other unit...fair enough)
Situation-we have had earth electrode installed with a reading of 6.8ohms at earth bar.
Bonding to structural steel, water service and gas service is 50mm
Metering tails 95mm
Have fitted 12 way memsheild 3 metal clad dist board with 200 mccb incomer with earth leakage relay, shunt trip and ct.'s adjacent to intake position. This incomer is only available as 3 pole.
100ma single pole rcbo for emergency lighting, fire alarm, small power
30ma single pole for sockets
No three phase circuits at this stage

Problems being M and E consultant is satisfied with 3 pole automatic disconnection..I maintain it should be 4 pole.

I intend to install 4 pole 200amp insulated isolator after metering and before dist board ..(if I can get one) and use paxolin inside trunking to eliminate possibility of earth fault before earth leakage relay.

I wanted to install 4 pole mccb with earth leakage relay but
Had a quote for 1k and m and e consultant has said it is not necessary.

Is three pole automatic disconnection ok?
Is paxolin in metal trunking acceptable to eliminate earth fault before rcd? Or should I change to PVC trunking (doubt it will will rigid enough for 95mm singles around bends)
If three pole automatic disconnection is ok do single pole rcbo need to be double pole?
Products at 100amp 4 pole rccb seem to be readily available but 200amp is proving a challenge

This is first time involved in tt sernario

Thanks in advance
Nick
 22 July 2012 12:22 PM
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broadgage

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To reduce the risk of an earth fault upstream of the RCD, I would either use insulating trunking, or metalic trunking with double insulated conductors within instead of the usual single covered conduit cables.

I suspect that in practice an earth fault upstream of the RCD would operate the cut out fuses, though you cant count on this.
Although the earth rod resistance of say 7 ohms would appear to limit the fault current to about 30 amps, in practice the fault current will be much greater since there is a low resistance connection to the structural steelwork which is both a good earth electrode in itself, and is also connected to the PME terminal of the other supply.
 22 July 2012 01:57 PM
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perspicacious

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Welcome Nick

"Had a quote for 1k and m and e consultant has said it is not necessary."

Can we assume that you've presented your client with an EIC with the site address on and he's got the "consultant's" signature in the design box and handed back ready for you to continue filling in the construction and I&T parts?

Or is it one of those build and then belatedly design projects?

Regards

BOD
 22 July 2012 02:07 PM
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21stcentury

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Hi BOD, belated d and b, as you've guessed.

Was no issues until supply was tt'd.

Thanks Nick
 22 July 2012 02:46 PM
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slittle

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Both Hager and Merlin do a 200 amp 4 pole MCCB based earth leakage device.

Can't remember the part number off hand but they work fine in TT situations other than the enclosure is metal (but there's ways and means around that)

I'm not so sure a fault upstream would clear the cutout fuses broadgage, it's more likely to start pulling the installations MET up towards 230 volts, certainly not a risk I'd want to take.

Stu
 22 July 2012 03:17 PM
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weirdbeard

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Originally posted by: 21stcentury

Hi all, this is my first ever post on anything like this so just dipping my toes in.

Background- existing large retail unit has been spilt into 2 retail units. 1 unit is utilising the existing 400amp Three phase TN C S supply. The other unit is now being supplied by 200amp TT (sse wouldnt provide earthing connection as the have steel members are common to the other unit...fair enough)


Hi Nick, seems odd the electric board won't supply a TNC-S earth terminal to this unit. Although you have an electrode and earth leakage trip I would say that the second unit can't be classed as TT as the second units earthing arrangement is surely directly connected the the suppliers neutral via the common steelwork and bonding conductors - this suggests to me that it would be more appropriate for the second unit to also be TNC-S?
 22 July 2012 03:32 PM
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alancapon

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Originally posted by: 21stcentury
. . . Was no issues until supply was tt'd. . .

In this scenario, the second supply was always going to be a TT. I know of no DNOs that would provide a second TNC-S into the same building (or one that shares structural steelwork with another). The problem is that if the neutral fails on the TNC-S supply, then if the second supply is also TNC-S, then the structural steelwork will carry the neutral current and hides the fault in the supply network.

You need to be careful of your single pole rcbos - depending on the nature of the fault, it is conceivable that neutral-earth current will still operate the main incomer, even after the rcbo has operated.

Regards,

Alan.
 22 July 2012 07:40 PM
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weirdbeard

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Originally posted by: alancapon



In this scenario, the second supply was always going to be a TT. I know of no DNOs that would provide a second TNC-S into the same building (or one that shares structural steelwork with another). The problem is that if the neutral fails on the TNC-S supply, then if the second supply is also TNC-S, then the structural steelwork will carry the neutral current and hides the fault in the supply network.





Seems a resonable explanation, but isn't this the reason why BS7671 requires larger bonding conductors for PME, to cope with such diverted neutral currents?
 23 July 2012 10:46 AM
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perspicacious

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"Seems a resonable explanation, but isn't this the reason why BS7671 requires larger bonding conductors for PME, to cope with such diverted neutral currents?"

Are you assuming that every "earthed" conducting path in the installation will be capable of carrying the magnitude of diverted neutral current?
The gas and water pipes might be but what about any screened cable for comms or fire alarm for example?

Regards

BOD
 23 July 2012 01:52 PM
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AJJewsbury

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The problem is that if the neutral fails on the TNC-S supply, then if the second supply is also TNC-S, then the structural steelwork will carry the neutral current and hides the fault in the supply network.

Two TN-C-S supplies in the same steel framework also means that a significant proportion of the installations' N current in normal conditions would flow through the steel frame to the cutout nearest the substation - which can have some nasty EMC effects. I worked at one place where the images on CRT monitors in one corner of the unit would always "wobble" - only years later did we find out why.

Problems being M and E consultant is satisfied with 3 pole automatic disconnection..I maintain it should be 4 pole.

Single/3-pole probably OK for automatic disconnection (after all if Zs was low enough you'd be permitted to use fuses), but not for isolation on TT (537.2.1.1) and a main switch is meant to provide isolation (537.1.4)

- Andy.
 23 July 2012 02:34 PM
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21stcentury

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Hi all, just to recap the 3 pole automatic disconnection set up

200amp 3 phase tt supply incoming

earth electrode installed
(6.8ohms at earth bar)

bonding to structural steel, gas and water

95mm metering tails in PVC trunking or double insulated in steel trunking

insulated 4 pole 200amp isolator

3 pole 200amp mccb with adjustable earth leakage relay incomer (set at 500ma) in metal clad dist board

Single pole rcbo at 100ma for general circuits
Single pole rcbo for 30ma for socket outlets

good to go... without the need for 4 pole automatic disconnection or double pole due to low Zs.

Thanks nick
 23 July 2012 02:46 PM
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AJJewsbury

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I'd still ponder Alan's point that although a N-PE fault will trip out the final circuit's SP RCBO, it won't disconnect the fault, hence there's a definite risk of the main ELR tripping out too (after the set delay).
- Andy.
 23 July 2012 06:32 PM
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alancapon

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It is also conceivabe that a phase to earth fault will still trip both devices. The local single pole rcbo due to the phase-earth fault, and the main incomer as a neutral-earth fault using the load resistance to the original fault to give the neutral to earth connection.

Regards,

Alan.
 05 November 2012 07:46 PM
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weirdbeard

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Originally posted by: AJJewsbury

I'd still ponder Alan's point that although a N-PE fault will trip out the final circuit's SP RCBO, it won't disconnect the fault, hence there's a definite risk of the main ELR tripping out too (after the set delay).
.


Originally posted by: alancapon

It is also conceivabe that a phase to earth fault will still trip both devices. The local single pole rcbo due to the phase-earth fault, and the main incomer as a neutral-earth fault using the load resistance to the original fault to give the neutral to earth connection.




Hi all, sorry to pop back to this past topic, but been giving this matter some consideration recently.

If we add in a 30mA single pole RCBO for additional protection to a circuit that is part of a TT installation with a main upfront device, does this mean that its considered that adding an additional protective measure is going to increase the likely-hood that the upfront device will trip?

To try and explain what I mean, on a farm some kind of appliance could be hard wired and only have the protection of 300mA RCD, but the same appliance, if it were plugged into a 13A socket would be required to be 30mA rcd protected-

I think my point here is: without the additional protection added, if there's a neutral to earth fault, the main trip would have gone anyway, so if there's any problem in this happening( in terms of circuit division)it is not down to the single pole RCBO that may have operated to give additional protection, but down to the up front RCD device being a problem......



Any thoughts?
 06 November 2012 12:31 AM
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alancapon

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Originally posted by: weirdbeard
. . . If we add in a 30mA single pole RCBO for additional protection to a circuit that is part of a TT installation with a main upfront device, does this mean that its considered that adding an additional protective measure is going to increase the likely-hood that the upfront device will trip? . . .

I believe that it does not affect the likelihood that the main device will operate.

I can argue the case that under a number of circumstances, choosing a 30mA single-pole RCBO offers few advantages over a straight mcb (ignoring for the moment that a 300mA device is not suitable for protecting life).

Consider phase to earth fault, on a TT system with a phase - earth loop resistance measured on the rod of 10Ω. The earth fault current will be 23A, which will operate the RCBO.

If the load connected to this tripped circuit was 6kW (for example), it will have a resistance of 8.8Ω. We now have a neutral - earth fault as the RCBO only opened the phase. The circuit is earth --> fault --> load --> system neutral. With a neutral to earth voltage of just 6V in the installation, the earth fault current through the new fault circuit is 319mA. This should open the incomer.

Regards,

Alan.
 06 November 2012 09:44 AM
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AJJewsbury

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Or ask your supplier for a double pole switching RCBO. They are made and are readily available elsewhere in the EU - I guess we just have to create the demand.
- Andy.
 06 November 2012 09:57 AM
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Parsley

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I think the standard RCBO doen't work in this situation and it would be wise to use either RCD's or double pole RCBO's (Hager manufacture DP RCBO's and Eaton make DP MCB's that can have their RCBO pod added).

GN7 advises the following:

All circuits other than socket-outlets must be protected by 300 mA RCDs for fire protection purposes. These will need to discriminate with final circuit RCDs where relevant.

For improved continuity of service, RCDs should be time delay or S type as long as they are not supplying socket-outlets.

Circuits supplying socket-outlets up to 32 A require protection by 30 mA RCDs. Circuits supplying socket-outlets exceeding 32 A require protection by 100 mA RCDs.

The schematic in GN7 does not have a RCD main switch just a main switch, individual submains are 300ma RCD protected and then the final circuits have either 100ma or 30ma RCD protection for socket outlets not >32A.

Alan

I can't make your maths work to get 6V between neutral and earth. I get 18.8ohms (efli+load) x 0.319ma= 5.99V, but can't work out how you got 0.319ma in the first place, can you clarify please, sorry if I being thick.

Regards
 06 November 2012 10:14 AM
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AJJewsbury

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I can't make your maths work to get 6V between neutral and earth. I get 18.8ohms (efli+load) x 0.319ma= 5.99V, but can't work out how you got 0.319ma in the first place, can you clarify please, sorry if I being thick.

I read it as Alan suggested an "arbitrary" 6V N-PE difference (e.g. due to v.d. (rise?) in the supplier N) and derived 0.319A (not mA) from that.
- Andy.
 06 November 2012 10:24 AM
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OMS

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I read it as "You would only need a 6V voltage difference between neutral and earth to call out the 300mA device - ie 0.319A"

Regards

OMS

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Failure is always an option
 06 November 2012 10:36 AM
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Parsley

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Thanks Andy, Yes that should have been 319ma not 0.319ma.

I get that the resistance in the neutral under load causes a vd and rise between neutral & earth but couldn't see how it would be 6V using the figures quoted.

Regards
IET » Wiring and the regulations » 200 amp three phase tt supply

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