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Topic Title: Rcd issue
Topic Summary: RCD not tripping on tt system
Created On: 04 November 2012 09:12 PM
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 04 November 2012 09:12 PM
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pixelmonster

Posts: 7
Joined: 08 July 2012

Hi, I went to test circuits at a cafe in a public garden. The system is TT on a 3 phase wylex board.Ze is 0.67. Ra is 1.07. Main earth was disconnected during test.
Circuits tested fine apart from the RCD tests. The 30ma RCBOs on the sockets trip at 19ms at x1 but not at x5 and the 100ma main switch wont trip at all under x1 100ma test.
My meter is telling me that the contact voltage is exceeding the
pre set limit of more than:confused 100v.
I recalled that due to higher Ze readings on tt systems the x5 test doesnt work.
Can anyone shed some light on this or am I looking at a fault somewhere.
Many Thanks. Confused in Cornwall!!
 04 November 2012 09:15 PM
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slittle

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Joined: 22 November 2007

Main earth disconnected during test ??

I assume you mean during Ze/Ra testing.

Is your meter capable of x5 at 100mA, some I've seen won't do it. But there's no reason a x5 test should work on TT, my megger does it fine.

Stu
 04 November 2012 09:20 PM
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pixelmonster

Posts: 7
Joined: 08 July 2012

Yes earth disconnected during Ze test. My tester wont do x5 at 100ma but it wont trip at x1. What could cause this and the rcbos not to trip at x5?
 04 November 2012 09:21 PM
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pixelmonster

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Just to avoid any confusion the RCD tests are with the earth re-connected.
 04 November 2012 09:22 PM
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slittle

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Is the 100mA RCD an S type (delayed) and therefore confusing the meter I wonder ?

Can't think why on the RCBO's at the moment, it's been a long week away from work.


Stu
 04 November 2012 10:50 PM
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John Peckham

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I cannot understand that as you have a low Ze/Ra so the meter should not be aborting the test having detected a high touch voltage.

Are you testing with the meter plugged in at a socket? Have you disconnected all the loads before testing? Did you do a Zs test at the socket before you did your RCD test? What meter are you using? Have you done your monthly confidence check on your meter to see if it is functioning correctly? How old/good are your leads? What make of RCBOs are they? Sounds to me your meter may not be generating enough test current?

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

http://www.astutetechnicalservices.co.uk/
 05 November 2012 01:40 AM
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pixelmonster

Posts: 7
Joined: 08 July 2012

Good questions!
The meter was plugged into a socket with no loads in the plant room. I also tried the test on one of the kitchen circuits. Loads were also disconnected.
I only did a Ze at the board.No Zs from the socket.
I am using a Unitest telaris meter.
No I havent done my monthly confidence check on my meter. It was calibrated in August.
Leads ok condition but 5 years old.
The rcbos are wylex.
The main switch is a wylex 63A 100A WRM63/4.
The supply comes from the gardens office.I disconnected the cafe supply from the main DB board there and did the rcd sequence of tests.0.5, x1 and x5 tests worked fine. I reconnected the cafe supply and it tested fine The earthing is PME.
I disconnected everything else from the office DB so only the cafe was connected to the main supply and x5 test on the rcbos and x1 on the 100ma rcd again failed to test.
I would like to retest the Ra to confirm the low reading.
Regards.
 05 November 2012 08:52 AM
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AJJewsbury

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Circuits tested fine apart from the RCD tests. The 30ma RCBOs on the sockets trip at 19ms at x1 but not at x5 and the 100ma main switch wont trip at all under x1 100ma test.
My meter is telling me that the contact voltage is exceeding the
pre set limit of more than:confused 100v.

Does the test abort with a contact voltage limit with the RCBOs at 5x as well as the 100mA RCCB?

So if the RCBOs trip OK at 30mA, but error at 5x30mA = 150mA, then the meter is suggesting that Zs is > 667 Ohms.

Likewise for 100mA 1x, the meter is suggesting Zs > 1000 Ohms. (both based on a 100V limit)

Ra is 1.07.

That seem very low for a simple electrode if parallel paths have been removed - no chance it's 1.07 k Ohms is there? (just asking the silly questions first)

- Andy.
 05 November 2012 10:41 AM
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leckie

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I am a bit confused by what you are describing, as you have said you have PME and TT. I presume you mean that the supply earth system is PME and that you have a sub-main that is TT? So what is the TPN RCD connected to the incoming supply or the sub-main?

The Ze value of .67 you quoted cant be for the PME (max .35), so I presume you mean for the sub-main. You said Ra is 1.07. Is it me, only I thought Ze and Ra were basically the same thing for a TT system? Or did you mean Zsb for the sub-main?

You also said everything tested ok, but then said you didn't test Zs at the socket - so if you didn't test it how can it have tested OK?

I must be missing something
 05 November 2012 10:48 AM
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AJJewsbury

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I thought Ze and Ra were basically the same

Technically, Ze is the impedance of the loop external to the consumer's installation - the consumer's earth rod is part of the consumer's installation - so Ze strictly speaking shouldn't include the resistance of the consumer's rod.

Hence statements like max Ze for TT installations is 21 Ohms - it includes the DNO's earth electrode, transformer windings and lines, but not the consumer's electrode - which usually the most significant part of the loop.

It's a pain really. Mostly we're just interested in Zs at the origin (regardless of the details of the earthing arrangement), but BS 7671 doesn't seem to give us a symbol for that
- Andy.
 05 November 2012 08:30 PM
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pixelmonster

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Hi Leckie, Sorry for the confusion.Yes the sub main to the cafe is TT which is connected to the TPN RCD. The 0.67 Ze reading is for this earth supply.
I took my R1+ R2 readings and added it to Ze for my Zs for the socket circuits.
I think the Ra is too low. I will return to the site and re-test.
Many Thanks
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