- #1
mrxybb
- 1
- 0
Can someone explain to me why voltage from Megger(500, 1000V) isn't that dangerous, why current is small?
Thanks.
Thanks.
Explanation about Megaohmeter (Megger)
- Thread starter mrxybb
- Start date
-
- Tags
- Explanation
Click For Summary
Discussion Overview
The discussion revolves around the safety and functionality of a Megger, specifically addressing why the voltage output (500V to 1000V) is considered safe due to its low current. Participants explore the internal mechanisms of the Megger and its historical context, as well as a tangential inquiry about sealing electrical cables through submarine hulls.
Discussion Character
- Technical explanation
- Exploratory
- Debate/contested
Main Points Raised
- One participant asks about the safety of the Megger's voltage output and why the current is low.
- Another participant attributes the low current to the large internal impedance of the Megger.
- A detailed explanation is provided regarding the original design of the Megger, highlighting its high output impedance and internal resistance, which limits current to safe levels.
- There is mention of the HighPot tester as an alternative used in industrial settings when a Megger is insufficient.
- One participant raises a question about how electrical cables are sealed through submarine hulls, prompting further discussion.
- Responses to the submarine hull question suggest that stranded conductors would need a strong water blocker, and a solid conductor might be more effective.
- A later reply suggests that the discussion about submarine hulls may be better suited for a separate thread.
Areas of Agreement / Disagreement
Participants express varying levels of interest in the Megger's safety features and its historical design, while the discussion about submarine hulls introduces a different topic that some participants feel should be separated from the main thread. No consensus is reached on whether to continue the submarine hull discussion within this thread.
Contextual Notes
Participants discuss the Megger's design and safety features without resolving the implications of its internal resistance or the specifics of submarine cable sealing. The transition to submarine hulls raises questions that remain unaddressed in the context of the original topic.
- #2
zoki85
- 1,198
- 230
Large internal impedance
- #3
sophiecentaur
Science Advisor
Homework Helper
- 30,403
- 7,470
The (original?) Megger consisted of a hand cranked alternator with many turns of wire on the coil - to produce the output of 500V, ( up to 5kV, for some models) The output impedance is deliberately made very high (by making th alternator 'not very good') so that it is 'safe' to use and also it is a non-destructive tester. The (series) internal resistance is several tens of MΩ and that limits the current that's available to a safe value. With only a fraction of a mA flowing through you, the output volts would drop to a safe level.
http://www.theautomationstore.com/megger/ The meter movement was cleverly arranged so that there were two coils, unlike the normal microAmmeter, which has one. The current into the test device was fed to one coil and the current through an internal reference resistor was fed to the other. The needle position ended up where the turning effect of the two coils balanced each other out. This 'ratiometer' arrangement gave a good measurement of the resistance under test, irrespective of how fast you turned the handle. (Smart, don't you think - for ancient Engineers with no access to solid state technology!?)
http://www.theautomationstore.com/megger/ The meter movement was cleverly arranged so that there were two coils, unlike the normal microAmmeter, which has one. The current into the test device was fed to one coil and the current through an internal reference resistor was fed to the other. The needle position ended up where the turning effect of the two coils balanced each other out. This 'ratiometer' arrangement gave a good measurement of the resistance under test, irrespective of how fast you turned the handle. (Smart, don't you think - for ancient Engineers with no access to solid state technology!?)
- #4
Doug Huffman
Gold Member
- 807
- 111
And when the megger wasn't enough, there was the HighPot tester in industrial facilities.
Did'ja ever wonder how electrical cables are sealed through submarine hulls?
Did'ja ever wonder how electrical cables are sealed through submarine hulls?
- #5
Bernie G
- 330
- 13
" Did'ja ever wonder how electrical cables are sealed through submarine hulls?"
Yes. Stranded conductors would require a strong water blocker. A bushing with a solid conductor might work better.
Yes. Stranded conductors would require a strong water blocker. A bushing with a solid conductor might work better.
- #6
sophiecentaur
Science Advisor
Homework Helper
- 30,403
- 7,470
A truly random (but interesting) change of course for the thread. Perhaps another thread?? I'm sure the mods would prefer it that way.Doug Huffman said:
