Device limit between System Sensor isolators

Notifier claims the ISO-X will not function properly if more than 25 devices are placed between isolators, while Silent Knight claims a maximum of 25 devices is “recommended” and System Sensor claims a maximum of 20 devices is recommended. Is 25 devices a hard limit, or is it simply a recommendation to increase survivability? If it is a hard limit, what issue occurs in the isolator that prevents it from functioning? In Style 4 SLCs, does this mean each isolated branch can only have a maximum of 25 devices?

25 is recommended for sure with iso-x, but ive seen alot more. 40-50. This is not a good idea.
The more devices you add the higher the current draw and more likely the isolation module will see the load as a short and open the circuit between the feed the load.
If your isolation module locks up or opens, its usually just on power up and you can usually remove the load wires and put them right back on while the feed wires are still on and itll be fine.
They should probably calculate this in current and not total devcies be different devices draw different current.
I would not recommend more than 30 on an isolation module.
The number of loops doesn’t matter. This # only applies to individual isolation modules.
You can have 4 loops with a dozen isolation module each as long as each iso is under 30.

The more devices you add the higher the current draw and more likely the isolation module will see the load as a short and open the circuit between the feed the load.

If too many detectors after an isolator can result in that isolator opening, how does adding another isolator help? Wouldn’t the first isolator still need to “carry” the current load of all devices after it, in addition to this newly introduced isolator?

The number of loops doesn’t matter. This # only applies to individual isolation modules.
You can have 4 loops with a dozen isolation module each as long as each iso is under 30.

By “loop” I assume you mean isolated SLC branch. How would this work? Would there simply be isolators placed in the middle of an SLC branch, despite the fact that a short that occurs before the isolator would still eliminate the entire branch?

hbt-fire-51253-U9.pdf (3.3 MB)

I attached the notifier SLC wiring manual.

By loops, its SLC circuits on the panels. I think your thinking of circuits or homeruns on the loops.
Say you have a 640 panel with an LEM card, now you have 2 loops. Loops 1 and 2. Now you can wire multiple isolation modules or isolation cards to each loop. Then you you add a 1 circuit or homerun to each output of each isolation module.
Unless youre talking about class A or style X SLC wiring then youre putting isolation modules between devices. That is not very common.
Usually you will see isolation modules at the facp or stacked in a riser configuration with dedicated homeruns of 30 devices or less pulled back to them.
It might be helpful to explain your application and the purpose of the question.

Usually you will see isolation modules at the facp or stacked in a riser configuration with dedicated homeruns of 30 devices or less pulled back to them.

So the 20-30 device limit does apply to isolated SLC branches on a Style 4 SLC. I was wondering for the purpose of combining multiple IDCs/SLCs into one larger SLC in a retrofit.

Unless youre talking about class A or style X SLC wiring then youre putting isolation modules between devices. That is not very common.

All of the warnings about 20 or 25 devices appear to refer to Style 6/7 circuits because they say the devices are “between” isolators, which is why was unsure if a similar rule needed to be followed for Style 4. Presumably, the intended action is to insert an additional isolator in the middle of a segment if there are too many devices between isolators, which is not possible in Style 4. However, I am unsure how the number of devices between isolators is relevant in the risk of an isolator falsely tripping in Style 6/7, since the amount of current flowing through any isolator should be the sum of the current drawn by all future devices on the SLC.

Okay, so your conterting an old convtentional to addressable and want to use old IDC wiring as SLC.
Im not sure theres an easy solution. Maybe runs that contain 30 or fewer can be reused but runs with greater than 30 may need to be re-circuited. You could probably re-use the wiring till you hit 30 devices, then pull new cable to the devixe after that for a new feed then use old wiring to continue the circuit.
At least it will be daisy chained (no T taps) so very easy to trace wiring.

Actually, according to the manuals for some isolators such as the Silent Knight SK-ISO (but not the SK-ISO datasheet or the manual for the Notifier ISO-X), there is an “isolator load rating” for each SLC device, and the sum of isolator load ratings between isolators must not exceed 1. Most System Sensor devices do not have stated load ratings in the manuals, but the 2351 (as well as the FSP-951) has an isolator load rating of 0.0063, meaning a total of 158 FSP-951 detectors can be placed between isolators. This could make it safe to place 40-50 devices between isolators if the right combination of devices is used.

If the 25 device limit is converted to isolator load ratings, then each device with an unspecified isolator load rating could be assumed to have a rating of 0.04 (1/25). Based on the exceptions to this rule, a B224RB would have a rating of 0.1 and an IPX-751 would have a rating of 0.48.

I am still unsure how an isolator would falsely open if too many devices are between 2 isolators in a Style 6 loop, as each individual isolator only has a concept of its own feed and load. Also, the Notifier SLC wiring manual states that most Notifier panels contain internal isolators at the SLC terminals, which would mean if no external isolators are installed in a Style 6 loop, there could be 318 devices between the two internal isolators, which clearly exceeds the 25 device limit.

Sorry to necro this but new here and just wanted to chime in with experience. The result of too many devices between isolators is that the isolator (and any subsequent isolators) will fail to restore after the SLC loses power. I don’t understand the electronics of it - it was explained to me by the factory years ago.

When this happens, you have to go and disconnect and reconnect isolators one by one to re-energize each segment of the loop. Happened a lot up here in Canada when people starting converting old AM2020 DCLB systems into NFS2-3030 and then converting to DCLC and adding isolators piecemeal.

25 is not a hard limit, it’s what Honeywell will guarantee to work in the worst case scenario. That limit hasn’t been updated in almost two decades so it’s likely the limit is much higher with modern devices and loop controllers but they aren’t willing to spend the money and effort to get it re-listed by the labs or determine the exact isolator load of each device.

If you are converting conventional zones to isolated class b/style 4 circuits, the isolator limit is irrelevant (technically speaking - can’t speak to any code requirements) so long as you are not installing any additional isolators downstream. Notifier has a card specifically designed for this (ISO-X) for example.