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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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 06 November 2012 06:25 PM
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alancapon

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Originally posted by: Parsley
. . . 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. . .

The 6V is a function of the supplier's network, and the relative balance of phase currents within the distribution network. I needed something about that for my scenario to work, and to be honest, 6V would not be an unusually high figure. As a whole number, it also made the maths easier.

My point was over the use of single pole RCBOs following an RCD, and wanting to show that you can easily get unexpected results following a fault in the installation.

With a longish single phase run (the worst scenario) and a transformer terminal voltage of 249V (which would be about usual), you could have a teminal voltage at the supplier's meter of 236V (depending on load etc. etc.). This is a drop of 13V in the supplier's network. In my single phase example above, that would be a drop of 6.5V in the phase and a rise of 6.5V in the system neutral.


Regards,

Alan.
 06 November 2012 08:04 PM
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Parsley

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Thanks for the explanation Alan

Regards
 06 November 2012 08:06 PM
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weirdbeard

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Originally posted by: alancapon

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).



.


Hi Alan, thanks for the reply, though it probably wasn't clear, as i didn't explain myself well and probably won't again, but -, I wasn't questioning whether the main trip would go if there was a neutral to earth fault, I was more questioning the recommendations for double pole RCBOs over single pole ones, as in - is there anything wrong with fitting single pole RBCOs from a shock safety, rather than a division of installation point of view?
 06 November 2012 10:15 PM
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leckie

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No. You will just drop the whole installation out if you have a n to e fault. Nothing wrong with that is there
 07 November 2012 12:13 AM
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alancapon

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I am not sure how to answer the safety issue of a single pole RCBO on a TT supply.

A problem on the network or a nearby property with a missing / failed RCD can successfully earth a phase conductor, often without operating a fuse. The voltage between the phase with the fault on it and earth can be just a few volts. Where the problem starts, is that the neutral will then be approaching 240v above earth, and the two remaining phases 415v to earth. While everything should still work (as the phase to phase and phase to neutral voltages will all be correct), the neutral is definitely a dangerous conductor in these circumstances. It is conceivable that this situation can exist for a while before it is discovered.

Regards,

Alan.
 07 November 2012 04:51 PM
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slittle

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And it makes for a fun time when it does happen.... (not)

I really think that amendment 2 should include double pole RCBO's on TT supplies.

If memory serves as I can't be bothered to reach across the office for the book, the mainswitch has to be double pole (or 4 as the case may be) such that the neutral which may not be "as close to the installations earth" as it would be in a tncs situation can be isolated.


Stu
 07 November 2012 04:59 PM
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AcidBurn

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Hey everyone,

I've also get lost in this very same subject. Especially about the regulations present in the countries where I work (Turkey, Syria, etc.).

I sincerely recommend the following site for regulations in Turkey (It's originally is a security site or sth, but they often publish papers involving very rich resources for us.)

Prodaft Security and Electrics

Let me know if you have stucked in the same thing in Turkey. I'd be glad to be helpful.
 09 November 2012 08:06 PM
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weirdbeard

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Originally posted by: alancapon



A problem on the network or a nearby property with a missing / failed RCD can successfully earth a phase conductor, often without operating a fuse. The voltage between the phase with the fault on it and earth can be just a few volts. Where the problem starts, is that the neutral will then be approaching 240v above earth, and the two remaining phases 415v to earth. While everything should still work (as the phase to phase and phase to neutral voltages will all be correct), the neutral is definitely a dangerous conductor in these circumstances. It is conceivable that this situation can exist for a while before it is discovered.



Hi Alan , once again thanks for the reply, though I must admit I don't understand why this is a problem that might be related to why double pole RCBOs have been recommended over single pole ones, if there's any possible problems, wouldn't it apply to all types of protective devices, such as single pole mcb's, not just the RCBO's?
 09 November 2012 08:10 PM
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weirdbeard

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Originally posted by: slittle


I really think that amendment 2 should include double pole RCBO's on TT supplies.




Hi Stu, ditto the above post - why should RCBO's be double pole, why not mcbs too?
 09 November 2012 09:56 PM
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slittle

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Because on a TT supply, an MCB will be backed up by an RCD which must be a double if not 4 pole device. Therefore under earth fault conditions all poles are disconnected.

With a single pole RCBO, the neutral is still connected once the device has tripped and therefore the circuit still has a "live" conductor connected to it under fault conditions.

It's been a long week and the is open so I hope the post makes sense

Stu
 10 November 2012 04:37 PM
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weirdbeard

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Originally posted by: slittle

Because on a TT supply, an MCB will be backed up by an RCD which must be a double if not 4 pole device. Therefore under earth fault conditions all poles are disconnected.


With a single pole RCBO, the neutral is still connected once the device has tripped and therefore the circuit still has a "live" conductor connected to it under fault conditions.




Hi Stu, all due respect for taking the time on a friday evening after, but strictly speaking although an RCD is the prefered method, the regs do allow for there to be no RCD in a TT system, there could just be a fuse or breaker if a suitably low Zs is permanently and reliably assured (415.5.2, ii )

So this suggests to me that multi-pole disconnection is not a prerequisite requirement for fault protection devices, and indeed 531.2.1 does only mention that an RCD shall be capable of disconnecting all the line conductors of the circuit, whereas 537.2.2.1 requires that a device for isolation shall isolate all live conductors.

On this basis I am thinking that in the most common TT situations, if an RCBO is intended to provide fault protection, then a single pole satisfies the regs. If the same device is intended to be used for isolation purposes it would need to be double poled.

I think what I have been trying to get at is: in a straightforward normal TT system there might be board with a main RCD and MCB arrangement, the MCBs only disconnect the line conductors in the event of a fault - a single pole RCBO is surely no different?

Any thoughts gratefully received!
 10 November 2012 05:19 PM
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slittle

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I'll go with your thinking, like I said last night, it's been a long week and I'd had a glass by the time I posted.

I think you are right about isolation and disconnection under fault conditions.

The difference (I believe) both Alan and I are considering is that under fault conditions with a single rcd and mcb's you would expect the whole board to trip in the event of an earth fault (because the mcb's will never disconnect it) however with RCBO's, they will indeed disconnect the fault but if there's an upstream RCD protecting distribution, it could still see a N-E path and open as well causing isolation of parts you might not want to.

Perhaps the relevant sections and guidance notes need a tweak in the 18th so we all understand exactly what those in power are thinking
 12 November 2012 08:04 PM
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weirdbeard

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Stu, thanks for your thoughts again.

The reason I have been asking about this was I thought I might have been missing something when a couple of prominent posters recommend the use of double pole RCBO's over single pole ones which has also been mentioned a few times in previous topics - I was concerned that single pole devices caused some kind of problem that I did not know about, but it does just seem that the primary reason for the recommendation of double pole devices is a division of the installation issue where there is a single upfront device.

Cheers.
 12 November 2012 09:53 PM
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MAXMIRA

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Nick,

I would suggest you look at reg 412 and 433.3 and with that in mind go for double insulated tails into a double insulated combined MCCB/RCCB such as the FDB19.

Dave
 09 February 2013 12:27 PM
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21stcentury

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Hi all, it's been a while put finally had this installation energised on thursday (yes thursday 07/02/2013. It's been a painful experience but here goes with an update.

Decided to change original steel trunking to PVC and also install a 4 pole GRP 200amp isolator with earth leakage relay, current transformer and shunt trip to distribution board 200amp 3 pole MCCB.
Dno turned up to fit ct and connect tails, they decided they had fitted the incorrect ct chamber and refused to connect tails. The don had fitted a metal enclosure and had left the PME.
 09 February 2013 12:48 PM
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21stcentury

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Sorry sent that too early

200 amp tt supply

Decided to change original steel trunking to PVC and install 4 pole GRP isolator with earth leakage relay, current transformer and shunt trip to distribution board incoming 200amp MCCB.
DNO turned up to connect tails and fit cts...they decided that the incorrect ct chamber had been fitted by themselves and refused to connect tails. Also the Service head was connected as PME with N-E link.
They also refused to connect to isolator and insisted on connection to rcd only (incase a supply was taken off isolator with rcd protection)
After much coming and going they decided that both supplies needed to be TT, as they were not happy with a PME and TT within the same steel frame.(different earthing systems)
So we had to get the 4 pole rccb (got it from CPD in the end) and fit in GRP enclosure before isolator.
Original PME supply next door had to have upfront rccb fitted with earth electrode (tt)
Dno had to change there ct enclosure.
All carried out with ct guys turning up before chamber had been changed, metering turning up before ct.'s had been fitted, night shift to install rccb for next door and almost daily emails with various deadlines missed.
Drama over I think!
IET » Wiring and the regulations » 200 amp three phase tt supply

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