Hi Rick:
> Somewhat related to this discussion is the opportunity I had last week to
> perform surge testing on an off-the-shelf, rack mounted PC. The power supply
> in question was from a reputable manufacturer and had safety, EMC and CE
> markings. The system (power supply) passed the "Heavy Industrial" limits
> when tested line to neutral but the computer would reset when tested L/N to
> ground. My rationale for this anomaly was the lack of a suppression device
> (MOV?) to ground.
I can take some guesses as to what is happening:
1) The surge current path within the PC developed an
EMI radiated signal within the PC that initiated a
reset.
2) The PC was interconnected with other units; the
surge current path divided between the PC's ground
wire and the interconnected units' ground wires.
A voltage developed across the signal ground wires
(because they are smaller diameter and higher
resistance the protective ground wires) which
appeared to be a corrupt data signal. The PC reset.
(However, I would have expected corrupt data, not a
consistent reset, from this hypthesis.)
3) Same as 2, but an EMI radiated signal from the
grounding path caused the reset.
Whether or not the PC has a discrete suppression device,
the surge current MUST return to its source (i.e.,
complete the circuit). The suppression device limits
the voltage excursion, and probably actually increases
the current. So, I'm not at all sure that a suppression
device would prevent the reset
In the absence of a suppression device, the current is
due to the Y capacitors and stray capacitance in the unit.
Initially, the surge current will be high, but then,
through the capacitance, it can decay faster than with a
discrete suppression device. But, the Y-capacitors will
cause a double differentiation and there will be a
negative voltage swing before the surge event is complete.
A "suppression device" CAN be used to control the current
path through the PC such that the surge current does not
cause a reset.
By the way, Y capacitors can also set the surge current
path.
Best regards,
Rich
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