Why is daisy-chaining surge protectors not recommended?
You should not daisy chain protective devices (fuses, MOVs, breakers, etc.) without first doing the appropriate research because generally speaking, they are rated to interrupt in x seconds given a particular fault current. When you have two protection devices with similar ratings in the circuit, they end up both trying to interrupt and may very well end up interfering with each other's interruption capability, possibly to the point where neither will properly clamp or interrupt the fault, causing excessive current flow and possibly fires.
e.g. A fuse chosen more or less at random will clear in 1s with a ~20A fault current. If you have a second fuse with similar ratings in series, they will actually start limiting the fault current as they open up, and the fault current is no longer 20A, it may be 15A, or 10A, or ... you get the idea. That same fuse will clear a 10A fault in ~10s, which could be enough time to heat up wire or traces or cause a semiconductor to fail because it wasn't designed to handle that kind of current for that kind of time.
e.g. A MOV series chosen more or less at random will clamp a surge at 130V. Two in parallel will have (slightly or significantly) different clamping voltages, usually with the lower one "winning". The breaker/fuse and MOV are usually selected so that the MOV will clamp and the breaker will open with the surge current, but when you mix and match you end up with a MOV clamping earlier, which the fuse/breaker wasn't designed to trip at, which now alters its fault ratings, leading to unpredictable protection.
In the industrial power world this kind of interaction is actually a significant part of the overall electrical design, since you have substation transformers protected with fuses, and the downstream equipment protected with their own fuses or breakers, and then the load controllers protecting their semiconductors or motors again with their own protective devices, usually a combination of MOVs or fused MOVs and either breakers or fusing. There's a lot to look at, including the I2T ratings of the protective devices, interrupting capabilities, pulse withstand capabilities, temperature derating, clearing times, current limiting effects as the devices become active, Joule ratings and so on. Here are a few good references if you wish to look into it further. The term "fuseology" has come about to describe this particular aspect of electronics design.
... and I bet you thought fuses, breakers and TVS type devices were pretty simple, didn't you. :-)
Re a few of the responses, supposedly from knowledgeable engineers . . . .
Daisy chaining surge suppressors (running them in series) would not increase or decrease their current interrupting capability. The current is the same every where in a series circuit, so the first fuse or breaker would be exposed to the total demand of chained surge protectors, and it should interrupt at its rated capacity. No matter it was something plugged into that unit or one down stream.
Now if you place multiple protectors in parallel into the same outlet, the house breaker would be exposed to the total demand. Either way the smallest rated fuse device would pop first, as they should. No problem here either.
- I believe the liability factor has a lot to do with posting the "Do not daisy chain" warnings.
- Also, the internal nesting of the MOVs in parallel should be more of an issue than anthing. Even tho, the ratings of the MOV devices are the same, they all do not react at the same time, the first to fire, takes the biggest energy burst and would be the first to fail, and the next to activate would get the 2nd highest burst, and so on. Even tho, this happens in micro or milli-seconds, it still happens. However this is what is needed to build up large, electrical shock absorbers. Because MOVs are not made in large capacity, they must be bundled in parallel to get high ratings.
- Also, important to realize, is that the joule ratings on the box or device are the total capacity between all three leg-pairs. Therefore, the actual ability is one third between any two legs, at one time.
- As for aging, metal-oxide-varistors (MOVs) age like other electronic components. One way to check them, tho it's a bit tricky, is to unplug the arrestor from the mains first. Then setup a multi-meter set for about four to five MegaOhm range of resistance. Place the probes into the socket holes and look for any resistance between the three pairs, neutral to hot, neutral to ground, hot to ground. You should see OL (over-load) readings, which means the resistance is too high to register on this range. However, you may also see initial readings that then climb until they go OL. This is the effect of capacitance charging. If the readings just stay fixed at some Mega-ohm (or lower) reading then this is probably a leaky MOV.
I worked electronic-technical-support for major fortune 500 companies for 25 years. As the ability of surge suppressors increased, We always recommended a minimum of 2,000 joules. Now days, I would recommend at least 3 thousand joules and a clipping voltage of no more than 330 volts, preferably lower. Also the line cord should be at least 14 gauge, 12 gauge if really long. I recommend surge suppressors on all electronic devices and also on devices that have motors or compressors because they are the sources of most local spikes. This includes dish washers, washing machines, dryers, refrigerators, freezers, air conditioners, etc.The power grid in the U.S. has consistently become more and more dirty. It is now awash with all kinds of R.F. and other switching hash which travels collectively up and down the power lines between sub-stations.
Like belly-buttons, we all have our opinions. But, hopefully they are based on sound electronic theory. Later.
I believe daisy chaining is discouraged to prevent overloading of the circuit(s) and the warnings are more for minimizing insurance liability.
A consumer-grade power strip is good for 15 to 20A. Put another one on and the first unit is now expected to endure 30 to 40A. (and so on). Another reason people will daisy chain is to extend the reach of the electricity. As the power strip is intended for its own 6 feet of reach, the conductors are not suitable for 10~15A over the longer distance of 3 or 4 power strips of length. Instead of daisy chaining for length, buy a thicker gauge extension cord to fit a power strip to.
Obviously, everyone does this anyway. Me personally, I put computers on their own power strip then daisy chain all the odd power adapters as needed to get all of them fed.