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 Graphic demonstation of switching AC vs DC
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 Return to top of page · Post #: 1 · Written at 5:18:48 PM on 25 April 2017.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 5427

Switching DC can cause serious arcing, as graphically demonstrated here:

https://www.youtube.com/watch?v=Zez2r1RPpWY

... and further evidence as to why switches have different ratings for AC and DC:

https://www.youtube.com/watch?v=4kcwgPs-DBE

But high voltage AC (such as 500kV here) is also capable of serous arc flashing:

https://www.youtube.com/watch?v=o1LeDTl2GDg

And sparkies would (or should!) be aware of this danger, too:

https://www.youtube.com/watch?v=6hpE5LYj-CY


 
 Return to top of page · Post #: 2 · Written at 10:25:44 PM on 26 April 2017.
Samt's Gravatar
 Location: Hobart, TAS
 Member since 6 May 2013
 Member #: 1337
 Postcount: 72

I recently had the rare opportunity to visit the light station on Tasman Island. Tasman island is a remote island off the East coast of Tasmania that is surrounded by 300 metre high cliffs and is only accessible by helicopter now since the haulage way up the cliff used for boat landings fell into disrepair. The light house and cottages were built in 1906. No one has lived on the island since the light was automated in the late 1970's.
The three light house keepers cottages on the island were supplied with 110 volts DC for lighting from a bank of batteries that were charged by a generator and Lister engine. I had a look inside the cottages which had been abandoned for 30 years and the DC wiring for lighting was still in place. I noticed that porcelain pull cord switches mounted on the ceiling were used instead of the traditional Bakelite AC wall switches. The island is now part of the Tasman national park and the cottages are now being restored by volunteers after 30 years of neglect.
The 110 volt DC power was not used for the lighthouse as a pressurised kerosene lamp and clockwork mechanism was used for 70 years until a new automated light was installed. The automated light was powered by wind generators and later solar panels. The original light and clockwork mechanism is now on display at the maritime museum in Sydney.


 
 Return to top of page · Post #: 3 · Written at 10:46:49 PM on 26 April 2017.
Brad's avatar
 Administrator
 Location: Greenwich, NSW
 Member since 15 November 2005
 Member #: 1
 Postcount: 5726

DC at high voltages is a pain to switch and an even bigger pain to transmit because raising and lowering the transmission voltage is not a simple task like it is with AC. For those not familiar with AC, because the voltage reaches nought 100 times per second, the waveform provides a good built-in spark quencher. This does not exist in a DC transmission, where the voltage is always at the same level. Even in a humble vintage DC light switch, there are porcelain spark quenchers to break the current flow when the light is turned off. If you operate a standard HPM or Clipsal 'all-ways' mechanism on 240VDC it will self destruct when the switch is turned to the off position.

The upside with transmitting DC is that there are fewer electrical losses associated with it. The downside is that transformers will not work on DC. Motor-generator sets or electronics must be used instead.


‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾
A valve a day keeps the transistor away...

 
 Return to top of page · Post #: 4 · Written at 2:04:26 AM on 27 April 2017.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 5427

If you operate a standard HPM or Clipsal 'all-ways' mechanism on 240VDC it will self destruct when the switch is turned to the off position.

See the second YouTube clip above.


 
 Return to top of page · Post #: 5 · Written at 9:17:09 AM on 27 April 2017.
Robbbert's avatar
 Location: Sydney, NSW
 Member since 18 September 2015
 Member #: 1801
 Postcount: 1154

I remember hearing stories of DC disasters ages ago. For example, a light globe fused, and formed an arc which came out of the base and slowly worked its way up the twisted flex from which the globe was dangling. It would have got into the roof and started a fire if the switch hadn't been turned off.

No idea of the truth of this, it was just a tale I heard.


 
 Return to top of page · Post #: 6 · Written at 5:05:47 PM on 28 April 2017.
Brad's avatar
 Administrator
 Location: Greenwich, NSW
 Member since 15 November 2005
 Member #: 1
 Postcount: 5726

In theory it is certainly possible. The only saving grace is that back when DC was more common in households all the wiring was in split seam conduit.

The last DC mains in Sydney was turned off in the late 1980s and by that stage the last lifts and escalators that were supplied with DC would have either been fitted with rectifiers to change AC into the necessary DC or in rare cases, the motors and drives swapped for ones that ran on AC. These days all lifts and escalators have good AC drive systems and the better quality ones such as those by Schindler operate more smoothly than the DC units they replaced.

GTC's link to the video of the Clipsal switch in meltdown shows how easy it can happen.


‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾
A valve a day keeps the transistor away...

 
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