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Old 10th March 2008, 02:23 PM   #1
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Default Safety ground question

So, a very quick question (that, now that I'm done typing it, no longer qualifies as 'quick'):

I'm in the midst of restoring an old Zenith console radio/turntable, and have just finished recapping it. One thing that I know I need to do before I put any voltage on the power transformer is update it to more modern wiring standards - ie, the chassis (and power transformer's secondary CT) ought to be grounded.

Normally I accomplish this by just grabbing a fused IEC inlet, "making space" for it on the chassis with a hacksaw, and wiring it up in place of the original cord. The ground wire gets soldered to a convenient spot on the chassis to ground it all and I'm done.

However, it looks like in this case there's a 0.0047uF capacitor coupling one of the mains lines (this is from the days before polarized plugs in the US, I guess) directly to the chassis.

So, that's a lot of background to ask: Is it safe to just remove this dangerous-looking capacitor? I can draw a diagram if I've not been descriptive enough, but my MS Paint skills are sorely lacking.
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Old 10th March 2008, 02:44 PM   #2
PRNDL is offline PRNDL  United States
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>> Is it safe to just remove this dangerous-looking capacitor?

The short answer is "yes".

This cap is used to reference the AC return to ground, which is unnecessary and somewhat hazardous.

The AC hot and return should be wired to the power transformer leads, through a fuse and power switch.
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Old 10th March 2008, 03:06 PM   #3
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Quote:
Originally posted by PRNDL
unnecessary and somewhat hazardous.
That's what I thought. I saw the schematic, and the "whaa--!" alarm went off. Thanks!
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Old 10th March 2008, 03:11 PM   #4
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Do not solder the ground wire to the chassis. Use a screw/lug combination.

Make sure to use an "energy storage device" when fastening down the screw/nut. "Energy storage device" is a fancy name for a lockwasher, either star or split.

Cheers, John

ps..agreed, toss the cap..
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Old 10th March 2008, 04:21 PM   #5
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I've done many radio restorations, and there's nothing wrong with a line to ground capacitor (or across the line) provided it's rated for that use. Typically I just replace this capacitor with a modern one and it's good to go. I must admit that I'm more of a purist with regards to radio restoration though
Here's some good info on these types of capacitors:
ABC's of safety caps
Dave has excellent prices and hard to find values of caps.
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Old 10th March 2008, 05:20 PM   #6
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Quote:
Originally posted by jneutron
Do not solder the ground wire to the chassis. Use a screw/lug combination.

Make sure to use an "energy storage device" when fastening down the screw/nut. "Energy storage device" is a fancy name for a lockwasher, either star or split.

Cheers, John

ps..agreed, toss the cap..
Not doubting you, just wondering why? More (mechanically) secure fixing or better conduction?

Thanks,

James
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Old 10th March 2008, 05:35 PM   #7
AndrewT is online now AndrewT  Scotland
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Hi,
for mechanical security.
Due to the possibility of excessive heat inside a faulty appliance there must not be a soldered joint that could fail open circuit to defeat the Safety Earth protection.
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Old 10th March 2008, 05:38 PM   #8
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Quote:
Originally posted by jrevillug


Not doubting you, just wondering why? More (mechanically) secure fixing or better conduction?

Thanks,

James

Questions are ALWAYS allowed..

National Electrical Code 2008...page 70-97

250.8 Connection of grounding and bonding equipment.
(A) Permitted methods. Grounding conductors and bonding jumpers shall be connected by one of the following means.

(1) Listed pressure connectors
(2) Terminal bars
(3) Pressure connectors listed as grounding and bonding equipment
(4) Exothermic welding process
(5) Machine screw type fasteners that engage not less than two threads or are secured with a nut
(6) Thread forming machine screws that engage not less that two threads in the enclosure
(7) Connections that are part of a listed assembly
(8) Other listed means.


(B) Methods not permitted. Connection devices or fittings that depend solely on solder shall not be used.

Cheers, John

ps.. I notice I really didn't answer the question...sorry bout that..

For mechanical strength. Solder tends to ductile creepage over time, it's a connection of dissimilar metals that over time may corrode, no matter how you try to clean, there will be trapped flux within the joint. As a standard practice, never rely on any solder for the mechanical strength of a joint. And never "tin" a stranded conductor before clamping it under a screwhead...that always loosens over time.

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Old 10th March 2008, 05:39 PM   #9
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Quote:
Originally posted by AndrewT
Hi,
for mechanical security.
Due to the possibility of excessive heat inside a faulty appliance there must not be a soldered joint that could fail open circuit to defeat the Safety Earth protection.
I imagine the capacitors would already be toast by the time the chassis exceeded 183 degrees C... ...221 if it's lead free.

Cheers, John
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Old 10th March 2008, 06:13 PM   #10
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Thanks chaps.

James
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