All of the power supplies that we use have surge suppressors built in.
I'm sure that many manufacturers are shipping their products to the
Nordic countries with similar power supplies.
Darrell
----------
From: [email protected]
To: [email protected]
Cc: Rich Nute; [email protected]
Subject: Re: Surge Suppressors
List-Post: [email protected]
Date: Tuesday, May 13, 1997 12:58PM
Hi Ray:
In Norway, there is no guarantee that any particular outlet will
include a ground. At NEMKO's main meeting room, the outlets do
not have a ground!
Consequently, in Norway, the condition of no ground is a NORMAL
condition, not a fault condition. (Likewise, one-third of USA
homes have no ground, and all Japanese homes have no ground!)
So, Norway prohibits surge suppressors from being connected to
'earthed' parts of equipment.
Now that we've put that issue to bed, let's look at another
issue: Does a shock hazard exist because a surge suppressor
is connected to grounded parts of equipment when the equipment
is not connected to ground?
Let's first define the surge. Can we assume the "standard"
1.5 x 50 impulse? If so, we are looking at a "surge" that
decays to 63% of its peak value in 50 microseconds. It decays
to nearly zero in 250 microseconds.
We can further define the surge amplitude as not exceeding 1.5
kV. This is the dielectric withstand value of the primary-to-
ground insulation.
Let's assume the surge is less than 1.5 kV peak for less than
250 microseconds. Is this hazardous?
Can the human body feel this pulse? Probably.
Can the human body be "frozen" to the product by this pulse?
No. "Freezing" requires continuous current through the body.
Can the heart be caused to fibrillate by this pulse? No.
Fibrillation requires the current to extend for the full duration
of the T wave, i.e., more than 200 milliseconds.
Can the human body be burned by this pulse. Yes. A current of
70 mA peak or more can cause a burn at 1 cm square contacts with
the skin. It is likely that the body impedance will be less
than 1500/0.070 = 21,428 ohms during this pulse.
Note that protection against electric shock by the GFCI (aka
ELCB, RCCB, RCD) provides protection by limiting the duration of
the current through the body, not by limiting either the voltage
or the current. When a GFCI operates, the subject gets the full
voltage and whatever current, but only for a short period of time.
In this way, it prevents fibrillation and "freezing" to the circuit.
So, I submit that the impulse, while it can be felt and may create
a burn, is not a shock hazard.
Now that we've looked at electric shock, let's look at whether the
impulse can appear on the grounded parts of the equipment when the
equipment is not grounded.
Ignoring the surge suppressors, and considering for a moment the
EMC filter capacitors, we have the following circuit:
L1 <...-------------+---------------------+
| |
| |
---+--- |
CY1 |
---+--- -----
| | | Rload
+-----------+ | |
| | | |
---+--- | -----
CY2 | |
---+--- | |
| | |
| | |
| | |
L2 or N <...-------------+-----------)---------+
|
PE (non-existent) <...------------+
|
| chassis
-----
---
-
If a surge appears between L1 and N, it will be divided in half
by the action of CY1 and CY2. So, only one-half of the surge
voltage will appear on the chassis.
If a surge appears common-mode between L1/N and PE, then the full
voltage will appear on the chassis because there is no current
path (because the ground is open).
We can replace CY1 and CY2 with surge suppressors. If a surge
appears between L1 and N, the surge suppressors will not be
turned on unless the suppressor voltage ratings are less than
one-half the applied voltage. In this case, the circuit response
to the surge remains the same as with the capacitors. If the
suppressors are not turned on, then the circuit still behaves the
same as with capacitors because of the capacitance of the
suppressors.
If a surge appears common mode, the surge suppressors will not
be turned on because there is no connection of the chassis to any
other circuit. The full surge voltage will appear on the chassis
just as with the capacitors.
I submit that:
1) There is no electric shock injury hazard from a surge, but
there may be a burn hazard.
2) If the unit incorporates surge suppressors and is not grounded,
the hazard is no worse than without the surge suppressors.
Best regards from San Diego,
Zorro Viejo (aka Richard Nute)
ps: This explanation doesn't solve your problem. You can't use
surge suppressors between mains and chassis in the Nordic
countries.
