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Let’s assume

  • bog standard two layer ESD mat with static dissipative top and conductive bottom layer, with resistance to ground somewhere between one mega-ohm to one giga-ohm and with both a mat and an operator grounded to a common ground point;
  • bog standard consumer or HEDT motherboard.

A) If I were to put a completely new motherboard straight from its retail package onto the ESD mat, would there be any risk of said mat draining CMOS battery or shorting out RTC circuit powered by CMOS battery? Why? Why not?

B) Does the situation change if the motherboard have been on for some time before turning it off and immediately putting it on ESD mat? Why? Why not?

C) Is it safe to turn the motherboard on while laying on the ESD mat? Why? Why not?

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A) If I were to put a completely new motherboard straight from its retail package onto the ESD mat, would there be any risk of said mat draining CMOS battery or shorting out RTC circuit powered by CMOS battery? Why? Why not?

The mat typically has a resistance > 1 MΩ and < 1 GΩ. Such huge resistance won't short or (really) drain any battery.

B) Does the situation change if the motherboard have been on for some time before turning it off and immediately putting it on ESD mat? Why? Why not?

No. Same reason as in A. The resistance is huge enough not to cause damaging currents anywhere on the motherboard.

C) Is it safe to turn the motherboard on while laying on the ESD mat? Why? Why not?

Yes. Assuming the power supply providing the voltages to the motherboard is safe, the voltages on the motherboard itself (12V, 5V and 3.3V) are SELV, Safety Extra Low Voltage.

Important thing regarding A and B:
If you don't wear a ESD strap yourself (which has the same connection as the mat), you might damage the motherboard by the act of putting it on the ESD mat or, when it is already on the mat, by the act of touching it.
The second action is most easy to understand. There can be an Electrostatic Discharge (ESD) from your fingers through the air to the motherboard which drains away even better due to the ESD mat.
enter image description here Image source

When holding the motherboard without protection and approaching the map (to put it on the mat), this same ESD pulse can go through your fingers, through the motherboard, through the air, through the mat, which may have the same catastrophic result.

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A) of course it is safe. It's an ESD mat. Most likely battery self-discharge is higher than the mat resistance would cause. It is just meant to set the potential of the motherboard slowly to ground potential. It does not short out batteries or power supplies.

B) No difference. Still safe. Powering a motherboard does not change it's potential or accumulate charges to it. Power supply is at ground potential like the mat is.

C) Yes it can be turned on, the mat has extremely high resistance so it does not short anything.

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Conductive, dissipative, and insulating have very specific meanings. Your concerns apply to surfaces that are conductive, not things that are dissipative. Although, very very low power or high impedance circuits might have issues on a dissipative mat since the relative resistances between the two are no longer negligible.

enter image description here https://transforming-technologies.com/esd-fyi/difference-between-conductive-dissipative-and-insulative/

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  • \$\begingroup\$ What would be some typical examples of either very very low power or high impedance circuits used in consumer electronics? \$\endgroup\$
    – vrsio
    Aug 4, 2019 at 9:50
  • \$\begingroup\$ @vrsio Usually analog circuits for really sensors with really high output impedance (and therefore require a circuit with even higher output impedance since you usually want the output impedance of the sensor to be negligible compared to the input impedance of the circuit it is driving). I don't think you run into many in consumer electronics because they are tricky and expensive to design for. Pyroelectric sensors are probably the most common one and in that case all the tricky stuff is taken cared of by integrating it into the sensor element so it can be cheaply and easily produced. \$\endgroup\$
    – DKNguyen
    Aug 4, 2019 at 16:21

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