EEVblog #51 - Mains connected ESD

[DanB]

Hi Dave,

Thanks heaps mate for an awesome vblog! I have seen several of your most recent vblogs and loved them that much, that I have started watching them from episode 1 and will continue to do so until I am up to date. You're a legend! Love ya work!

In EEVblog #51 between around 9:35 and 9:46 you talk about your EDS binding points that goes down to "mains earth".  I am not familiar with the particular EDS binding post you mentioned but when you mentioned that it goes down to "mains earth", it priced up my ears. Pls, oh pls, tell me that it is not just a simple case of a wire that is connected to the same earth grid as your house!

Given your amazingly and broad knowledge and experience in electronics, I'm sure it's not the case but damn, it would make for a great vblog episode if you haven't already done so: "The dangers of using mains earth directly!"

I'll also suggest it in the "suggestions" forum also.

If you have already discussed this well, then, I'll go back and hide in my corner and watch some more vblogs

Cheers,
Dan - Perth WA.

[Zbig]

  That's exactly how you do it... 

[eV1Te]

You are correct that the earthing wire or even the water pipes in a home or other installation can become "live" with mains voltages and in several cases can kill the person discovering this, if the neutral wire coming into the building/fuse-box is somehow cut/damaged. This might not even be noticed at first as the current can go down to the ground through existing water pipes instead. But if someone does maintenance on the water pipes and removes a joint, suddenly live mains is fed into one of the pipes and the only way to ground is through the plumbers body.

Similarly this can happen for the earthing terminal in the wall if the incoming neutral wire is cut/damaged.

However the ESD terminal you use contains a large resistor (10 Mohm?) in series with the connection to mains earth. So even if under a fault condition where the earthing wire is fed with mains voltages, not enough current would pass through the person using the ESD terminal.

[DanB]

Cheers for info, eV1Te 

[ebastler]

You are correct that the earthing wire or even the water pipes in a home or other installation can become "live" with mains voltages and in several cases can kill the person discovering this, if the neutral wire coming into the building/fuse-box is somehow cut/damaged. This might not even be noticed at first as the current can go down to the ground through existing water pipes instead. But if someone does maintenance on the water pipes and removes a joint, suddenly live mains is fed into one of the pipes and the only way to ground is through the plumbers body.

Similarly this can happen for the earthing terminal in the wall if the incoming neutral wire is cut/damaged.

However the ESD terminal you use contains a large resistor (10 Mohm?) in series with the connection to mains earth. So even if under a fault condition where the earthing wire is fed with mains voltages, not enough current would pass through the person using the ESD terminal.

This is an unusual explanation for the series resistor. I would argue that in any electrical safety concept, one always trusts the earth wire to provide proper safety earth. If the earth connection were to become "live", the grounded metal chassis or enclosure of any electrical appliance could kill you, and there are certainly no series resistors in these.

The correct explanation for the series resistor in an ESD ground, in my opinion, is:
(a) It limits the ESD discharge current and thereby protects your DUTs.
(b) It limits the current in case an operator wearing an ESD ground strap touches some other, live wire, and thereby protects the operator.
See e.g. this discussion: http://www.circuitnet.com/experts/87211.shtml

[nanofrog]

In EEVblog #51 ... you talk about your ESD binding points that goes down to "mains earth".

He's talking about something like a Desco 09740 Common Point Ground for wrist straps. You can find similar versions on eBay at varied price points (example).

I am not familiar with the particular EDS binding post you mentioned but when you mentioned that it goes down to "mains earth", it priced up my ears. Pls, oh pls, tell me that it is not just a simple case of a wire that is connected to the same earth grid as your house!

Actually it is.  You're meant to use the electrical grounds, not water pipes, as they may not be sufficiently conductive (i.e. all plastic, or may be shallower if than the ground rod depth per building codes if metal <important regarding ground path resistance>).

In the US & CAN, you simply attach the ring terminal attached to the wire on the common point ground point block to a receptacle with a wall plate screw (this assumes the box or wall receptacle is grounded properly = wall plate screw will be earth grounded). Just make sure you've scraped off any/enough paint from the back side of the screw to insure a good connection.

[@rt]

Everything in the system is resistive including the mat and ESD foot strap and conductive coating on the floor to prevent a spark.
The floor coating and ESD mats are tested with a megga tester, and the electronic wrist strap tester you should
be using daily at the production entrance probably also fails the strap if it’s too short.