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Old 04-14-2009, 07:05 PM
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Keith Keith is offline
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Question Megger Amp Clamp

So I have been using the multi-conductor Megger clamp for taking readings on power for servers in my cabinets and I am seeing discrepencies between what the metered APC strips are reporting and what the sum of all of the amp readings from the megger are giving. The difference in the actual usage is about 6-10a per strip (overall on a 60a 3-phase circuit).

I am seeing a ton of 0.3 and 0.7 readings; however, there are too many for me to actually trust.

Has anybody actually successfully and accurately clamped a rack of servers with the megger? I am beginning to suspect that it is getting confused because it is so close to other power cables (somewhat of a rats nest in certain racks). I am making sure that the cable is 100% lined up and centered within the clamp.

Also, I am using the 1st setting (the circle with 3 conductors).

I have clamped around 100ish servers or so and the results are similar across the board.

--Keith
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Old 04-15-2009, 08:53 AM
Schumie Schumie is offline
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Hi Keith,

The problem with clamps is you are taking into account all 3 cables, as opposed to just the live which can give you very large variances as you have found due to a number of factors.

Also, I've found that clamps generally aren't very accurate on very low draw (anything that is less than 5 - 10 amps) and therefore aren't much use.

Ideally, you need to isolate the live wire to get as accurate a reading as possible.

I'd recommend that before putting a system live that you plug it into one of the plug style watt-o-meters, and that way you can get a more accurate reading before putting it in the rack.
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Old 04-15-2009, 02:46 PM
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Keith,
I've concluded after using this thing for awhile that it's just not very accurate. I've gotten very different readings re-measuring the same cable 2-3 times. For dependable precision, Schumie's suggestion is best: measure the hot wire(s) with a clamp. For rack measurements, rack PDUs with built-in meters are great. For retrofits, something like Wiremold's Meter Reader is a possibility.


Ken
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Old 04-15-2009, 04:13 PM
cernst cernst is offline
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Quote:
Originally Posted by KenB View Post
Keith,
I've concluded after using this thing for awhile that it's just not very accurate. I've gotten very different readings re-measuring the same cable 2-3 times. For dependable precision, Schumie's suggestion is best: measure the hot wire(s) with a clamp. For rack measurements, rack PDUs with built-in meters are great. For retrofits, something like Wiremold's Meter Reader is a possibility.


Ken
Ken, have you found antyhing like the WireMold product that is networked? that's something that I've been looking for for a while. ServerTech will not build me any.
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Old 04-15-2009, 05:18 PM
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Originally Posted by cernst View Post
Ken, have you found antyhing like the WireMold product that is networked? that's something that I've been looking for for a while. ServerTech will not build me any.
The Wiremold device uses a serial interface to talk to the display. Might be reverse-engineered, if someone has sufficient interest.

Alternatively, I've had one of these for a few years: Smart Works - Smart-Watt. The user interface is rough, but it's usable. Dunno if the company is still in business or if they have improved the product at all.

Another cheap alternative is Watts Up's .NET device: https://www.wattsupmeters.com/secure/products.php, although it's not clear if you must use their online service to access it.

Finally, Geist has this cool looking Inline Ethernet Meter, but I don't know much about it: Geist Manufacturing - Inline Monitoring.


Ken
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Old 04-16-2009, 10:16 AM
Schumie Schumie is offline
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For full rack monitoring in your own data centers, I'd recommend doing it at the disti board with something like the following...
Modbus -> SNMP Gateway
BACnet IP##to SNMP##Gateways:##Control Solutions

PowerLogic BCM
PowerLogic - Submetering

That way you can monitor everything by SNMP and voila
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Old 04-21-2009, 01:15 PM
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Be careful with the Powerlogic stuff. Some of their so-called "SNMP" meters require expensive, proprietary software for reading meter values because of secret protocols.

Ken
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Old 04-21-2009, 02:49 PM
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Quote:
Originally Posted by KenB View Post
Be careful with the Powerlogic stuff. Some of their so-called "SNMP" meters require expensive, proprietary software for reading meter values because of secret protocols.
Thats why it's important to get the ones with ModBus support, and then buy a seperate ModBus -> SNMP gateway
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Old 04-21-2009, 03:00 PM
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But, unless the Modbus commands are public, all the SNMP in the world won't help.



Ken
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Old 04-21-2009, 05:04 PM
Schumie Schumie is offline
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I was always under the impression that ModBus was an open standard?
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