Ground referencing DC voltage standards - Page 1
Forum section: electronics / general

author: Zenwizard
author_rank: Regular Contributor
timestamp: March 15, 2021, 04:20:28 pm
content: I have a fluke 343A and an EDC 1030 for doing some meter calibrations for the lab. The highest accuracy meter in the lab is a Keithly 7510. When I have checking in the EDC I was getting weird over load on the meter 6500 not the 7510 it Is fine. I found out that the EDC was floating to quite high a voltage above earth, grater then the 500V that the Keithly could handle on the low pin. Grounding the low side of the EDC allowed me to do the check in measurements I need to find that the Cal was quite off but the stability was there. I am curious if there is any draw back from just leaving the ground strapped and referencing the low terminals to ground on the EDC or the fluke. At least in terms of meter calibration.Zen

author: Dr. Frank
author_rank: Super Contributor
timestamp: March 15, 2021, 05:34:14 pm
content: Usually the output of calibrators is floating, i.e. fully isolated from Ground, meaning either mains ground, or case ground of the device.Same goes for the input of DMMs.Fluke instruments usually have a separate Guard and Case ground output, which can be connected to the output minus, if necessary, e.g. to mitigate ground loops.The 343A has a mains / case ground only, which can be connected to either output minus or plus, up to 500V.If your EDC 130 (which I have no experience on) shows some mains caused offset, its output minus may directly connected to case/mains ground.Check in the manual, how to interrupt this. I assume this to be a defect.Also check your 6500, if its minus input is somehow connected to mains , which would also be a defect.If you see an overload, maybe your mains in your house has a leakage problem with earth / ground versus "hot" connection, but it's quite obvious, that I'm not that familiar with the American mains system.Frank

author: Zenwizard
author_rank: Regular Contributor
timestamp: March 15, 2021, 06:02:47 pm
content: The low on the Keithly is not "directly connected to mains" It is allowed 500 VDC offset and will still work to spec. The main Issue I had was the EDC/ Kron Hite. I got 550Vdc between output low and chassis ground. which is what caused the Keithly to kind of freak out (display overload). In terms of trust the keithlys are new to me from Keithly but the EDC and the fluke are 2nd hand so my trust level is much higher that the Keithlys are not defect it would be in the EDC if there was any. I should mention that there is no current here. And a quick shorting bar is enough to bring the readings in to at least the range of sanity. From reading the manual it looks like bench meters are not totally isolated from mains earth like a battery meter is but I will admit I am but a humble tech and am out of my depth in a cal lab but learning fast.But it seems to me that a grater then 500Vdc off set from chassis ground is a huge amount to float at least in terms of the EDC. I have replaced the filter caps in the EDC but not much else in terms of repair work as it was working but it might be time for a total recap as there might be a cap leaking. I was checking the 1Kv range when it happened around 750V

author: alm
author_rank: Super Contributor
timestamp: March 23, 2021, 07:45:37 pm
content: There is a user's manual for the likely closely related Krohn-Hite 1030Ahere. Unfortunately the service part is missing. The specs say this about the output terminals:Code:ISOLATION: The output terminals may be floated 1000 Vdc and the primary isolation is 2000 Vac rms.OUTPUT TERMINALS: Floating output with case ground terminalBased on that, the offset is not normal. How much current is flowing between the output terminal and ground? What happens if you connect a 100 kOhm resistor between output terminal and ground? What is the voltage across the resistor?It sounds to me like there's some leakage, possibly through a capacitor between the output and the 1000V rail, inside the 1030.