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Topic Title: IT Problem after Earth Loop Testing Nearby
Topic Summary: Did my Test Meter cause a problem?
Created On: 12 June 2013 03:15 PM
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 12 June 2013 03:15 PM
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keithredpath

Posts: 425
Joined: 30 March 2002

After doing an Inspection and Test in a Comms Room, I was informed by the IT Guy that they had a problem with their network.

All I done was to do Earth Loop Impedance Tests inside the Comms Cabinets on the 13 amp socket outlets that were not in use.

I set my Megger 1553 to "No Trip" and recorded the tests results.

Is there a chance I created a problem by carrying out these tests?

I would like to know for certain as I frequently work in Comms Rooms and am always compromised by what I can and what I cannot test and feel this is the least I can do without causing a disturbance.

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keithredpath
 12 June 2013 03:32 PM
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daveparry1

Posts: 6328
Joined: 04 July 2007

I don't see how you could have caused any damage by doing a loop test Keith, an insulation test could be a different matter of course!

Dave.
 12 June 2013 03:56 PM
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AJJewsbury

Posts: 11686
Joined: 13 August 2003

It depends on the meter, but some - even in 'no trip' - will draw 10-25A between L & N as part of the test, which on a relatively high impedance circuit might well have caused the voltage to drop and/or spikes be introduced which could be seen by near-by equipment.

If the network gear was critical it should have been fed by a UPS anyway which should protect it from such events - unless of course you were loop testing on the load side of the UPS.

Unless it's a pretty beefy UPS, I probably wouldn't recommend loop testing downstream of the UPS - no only does it risk defeating the protection for the supplied equipment, it doesn't necessarily give a meaningful result as the loop impedance will almost certainly change as the UPS switches between mains, battery and backup lines and/or tap changes to trim/boost voltages (in the case of line-interactive UPSs). Most rack mounted IT kit is so surrounded by supplementary bonding (via the metalwork) and multiple c.p.c.s that the speed of ADS isn't the first concern when it comes to shock protection.

- Andy.
 12 June 2013 04:37 PM
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AJJewsbury

Posts: 11686
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Just to add that we've found that some of the cheaper networking kit - especially the smaller boxes supplied by "wall warts" - seem especially sensitive to mains disturbances. We even had one little router that locked up when an auto transfer switch operated - which was rather disappointing. Professional kit (rack mounted with IEC mains connectors) seems much more tolerant.
- Andy.
 12 June 2013 06:02 PM
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keithredpath

Posts: 425
Joined: 30 March 2002

Just to Clarify

All these circuits were deemed "Critical" but none were backed up by an UPS.

Keith

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keithredpath
 12 June 2013 06:54 PM
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John Peckham

Posts: 7575
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Keith

It depends on your make and type of loop tester you used. Some loop testers (e.g Fluke) on the no trip range use a high current test for a short duration. It could be that this current to earth may have routed in part via the IT kit to earth rather than all straight down the CPC.

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John Peckham

http://www.astutetechnicalservices.co.uk/
 13 June 2013 11:16 PM
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gkenyon

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Originally posted by: John Peckham

Keith



It depends on your make and type of loop tester you used. Some loop testers (e.g Fluke) on the no trip range use a high current test for a short duration. It could be that this current to earth may have routed in part via the IT kit to earth rather than all straight down the CPC.
However, then so would a real fault . . . which may mean that there is an issue with the way earthing / bonding has been implemented in one or more racks. For example, I often see the rack itself bonded to the protective earthing, but no connection between PDU and rack to reduce current flowing in parallel paths to equipment. Or I see no additional earthing at all, which means that when you bolt Servers, Switches etc. into racks, they generate fortuitous earthing parallel paths all of their own.

Of course, it could also just be a coincidence, or another thing entirely.

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Eur Ing Graham Kenyon CEng MIET TechIOSH
 14 June 2013 10:52 AM
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Angram

Posts: 557
Joined: 23 March 2009

The same goes for surge protection devices firing, of course !

You shield the front end but can send the pike up the kit's rear end.
The spike has to go somewhere.

Personally I would not be doing ELI near IT kit and especially not within a rack. "Earth" can be part of the datalink.

Do ELI further towards the supply and the check continuity.

On the other hand testing procedures should be specified and agreed with the owners.

Quick addendum: The rack should be treated like IT appliances. Just because sockets are 13amp doesn't mean you are in fixed wiring. Usually there is a flex lead involved.


terence
 14 June 2013 02:15 PM
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AJJewsbury

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"Earth" can be part of the datalink.

Less so nowadays. While that's true of the old RS 232 generation of interfaces, modern TP ethernet, recent SCSI etc are all differential and don't rely on a common earth reference.

- Andy.
 14 June 2013 02:22 PM
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AJJewsbury

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All these circuits were deemed "Critical" but none were backed up by an UPS.

Not very critical then!

If they've chosen not to protect themselves from network faults, installation faults, transients from large equipment, lightning, planned & unplanned maintenance etc etc (which might well be a reasonable economic decision) then an electrician with a loop tester is a small additional risk they should be prepared for. Tell them it's an impromptu disaster recovery exercise! (and a mild one at that).

- Andy.
 14 June 2013 03:11 PM
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OMS

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Tell them it's an impromptu disaster recovery exercise! (and a mild one at that).


LoL - that made me smile -

Regards

OMS

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