They are all called shunt mode protectors. Typically destructive
transient seeks earth ground. Each layer of protection is defined by
its single point earthing. Primary protection is provided by a utility
and requires homeowner inspection:
http://www.tvtower.com/fpl.html

Secondary protection is the 'whole house' protector, et al - also
defined by its single point earth ground as demonstrated by:
http://www.erico.com/public/library/...es/tncr002.pdf

In each case, wire impedance must be kept low as repeatedly cited in
professional papers - which means short distance, no sharp bends, no
splices, not inside metallic conduit, etc.

When Orange County FL emergency response system suffered damage from
lightning, then reasons for system protector failure were repaired.
They fixed the earthing system:
http://www.psihq.com/AllCopper.htm

Why? Each protector is defined by what it connects to. Earth
ground. Those interior protectors claim to protect from what type of
surge? Well plug-in protectors don't even define protection for each
type of transient. Why let a consumer know it protects only from a
transient that is typically not destructive and that is made irrelevant
by protection already inside equipment.

Many plug-in protectors are undersized because that MOV vaporization
even promotes protectors to the naive. Meanwhile, if better protection
is required, did Orange County install plug-in protectors? Of course
not. They enhanced earth so that effective protectors would perform
even better.

What does that protector inside a plug-in UPS do? Where in numerical
specs does it even claim protection? Is dBs filtering considered
protection? No. Do standard numbers such as C62.xx or UL1449 define
protection? No. It does not list protection from every type of
transient because you might notice a glaring anomaly. With no low
impedance connection to earth ground, it does not claim to protection
from surges that cause damage. They forget to define protection from
each type of transient so that a consumer might assume it. And then we
have this layer of protection - forgetting to mention what defines each
layer: earth ground.

Pete C. has a problem with low impedance wiring requirement when
virtually every responsible installation requires 'a low impedance
connection'. His own Square D 'whole house' protector is reported
earthed by 10 feet. Of course that protector could be made better if
earthing wire was shorter, no sharp bends, separated from other wires,
etc. Each protector is defined by quality of and connection to its
single point earth ground. As one industry professional defined it
for every building owner and operator:
http://www.harvardrepeater.org/news/lightning.html
The keys to effective lightning protection are surprisingly simple, and
surprisingly less than obvious. Of course you *must* have a single
point ground system that eliminates all ground loops. And you must
present a low *impedance* path for the energy to go. That's most
generally a low *inductance* path rather than just a low ohm DC path.

They are called shunt mode protectors. They don't magically stop
surges. As wire impedance increases, then surges find other
(destructive) paths to earth. Wire impedance defined by an equation
where wire impedance increases proportional to wire length multiplied
by a factor that includes the Log of 4 times wire length divided by
wire diameter. Wire length increases wire impedance twice over which
is why low impedance means a shorter connection. A fact demonstrated as
relevant by Thottappillil in "Electromagnetic Pulse Environment of
Cloud to Ground Lightning for EMC Studies":
The most important properties of lightning current that produce
damage are peak current, maximum rate of change of current,
integral of current over time, and the integral of the square of
current over time. ... In objects that present essentially inductive
impedances such as wires ... the maximum overvoltage is
proportional to the maximum rate of change.

Of course, higher impedance means higher maximum overvoltage when we
want voltage to earth to be as low as possible. So we carefully lower
that wire impedance connection to earth.

As wire gets longer, impedance increases unacceptably. Polyphaser
notes in TD1023: Multiple I/O port protection, Single Point Ground
considerations at:
http://www.polyphaser.com/ppc_ptd_home.aspx
Another complication in this scenario is the inductance of the
conductor between the I/O protector and the ground system.
The inductance will determine how much of the strike energy
is conducted into the ground system and how much is left to
elevate the transmitter chassis. Since strike energy is a high
frequency pulse, a low inductance path to ground becomes a
critical factor. Copper strapping is preferred over large
diameter wire as an inter-connecting media. Copper strap has
a large circumference and low inductance per unit length. The
strike energy, like water, will follow the easiest (least inductive)
path to ground.

So what does that plug-in protector adjacent to two TVs do? Leaves
both TVs at 8000 volts. Why? Wire impedance to earth ground is too
large causing TVs to sit at 8000 volts during a surge. Where is the
protection? Demonstrates how a plug-in protector - without a low
impedance connection to earth - does nothing useful.

Some hype a myth about layered protection and forget to mention what
defines that layer - earth ground. Why would supplemental protectors
with excessively high impedance somehow earth to a point that the surge
ignored when entering a building? If that earthing was made superior,
then a surge is better earthed by the 'whole house' protector. And if
earthing is not sufficient, then why is a supplemental (plug-in)
protector going to earth to that same insufficient earth ground?

Rather than waste money on plug-in protectors, what did they do in
Orange County FL? They repaired reasons for lightning damage -
insufficient earthing. Plug-in protectors are in the same protection
layer as a 'whole house' protector. If 'whole house' protector's
earthing is not sufficient, then a plug-in protector - with excessive
wire impedance - will somehow earth to same ground? Nonsense. And yet
that is what plug-in shunt mode protectors must do.

BTW, another solution called series mode protectors; beyond scope of
this post.

As has been repeatedly demonstrated, even in a paper from Martzloff,
et al:
... objectionable difference in reference voltages ...occur
even when or perhaps because, surge protective devices are
present at the point of connection of appliances.

Each layer of protection is defined by the and most essential system
component - earth ground. Primary protection layer is earthed at the
utility pole. Secondary protection layer is properly earthed adjacent
to 'whole house' protector. Since plug-in protectors don't even claim
such protection in numerical specs, then such protectors neither have a
dedicated wire for earthing nor do they even discuss earthing. A shunt
mode protector earths. No earth ground - such as with plug-in
protectors - means no effective protection. Better is to put money
into enhancing the earthing system.

Lower wire impedance connection to earth. Enhance the earthing
electrodes. Establish a single point earth ground to be used by all
incoming utilities. Money wasted on plug-in protectors is better spent
here. Effective protection is defined by its most essential 'system'
component - single point earth ground.

Pete C. wrote:
I understand wire impedance, you apparently do not and misapply the
concept. You haven't provided reference to any industry papers that
support your assertions.

Those burning power strip protectors due
to failing MOVs somehow are not fire threats?

Failing MOVs in an old poorly designed power strip / surge suppresser
combo that doesn't meet current standards could well be a fire threat,
however they have not been "vaporized" by a surge as you have claimed.
...

Apparently folks like you latch onto technical terms like "wire
impedance" without bothering to get an understanding of what it really
means and how it is not the same as the wires DC resistance. You have
also not provided a single link to your claimed "vaporizing" MOVs.

The answer to the OP's original question "It is dangerous to have more
than one surge protectors in you home I
have 3 surge protectors running in my home." is clearly "no", it is not
dangerous.

And as myself and others have indicated a quality "whole house"
suppresser is very beneficial, but it does not obsolete secondary
suppressers further down the line, nor does it obsolete suppressers on
non power lines like CATV, phone and data.

Pete C.