The classic symptom is a sounder at the far end of a long run that grumbles rather than sounds, or a beacon that flickers when the rest of the circuit operates. The device gets swapped, behaves the same, and the fault gets written up as intermittent. It usually is not intermittent. It is volt drop, and the circuit was borderline from the day it was installed.
Voltage drop is one of the few fire alarm problems you can predict entirely on paper before anyone lifts a lid. It is also one of the easiest to prove on site once you know what to measure.
Who this is for
Installers, commissioning engineers and service engineers designing, extending or fault-finding conventional sounder circuits and addressable loops. It assumes familiarity with Ohm's law and with reading a manufacturer's current-consumption table.
Experience level
Competent engineer. The calculation is simple; the judgement about worst-case conditions is where experience matters.
What voltage drop does to a fire alarm circuit
Cable has resistance. Current flowing through that resistance produces a voltage difference along the run, so the device at the far end sees less voltage than the panel is putting out. Every device on the circuit is fed through the cable that supplies the devices beyond it, so the drop accumulates as you move along.
Fire alarm devices certified to the relevant parts of the BS EN 54 series have a stated operating voltage range. Below the minimum, behaviour is not simply "quieter" — sounders can produce a distorted or weak output, beacons can fall out of their specified flash characteristics, and electronics can reset repeatedly. The device has not failed. It is being starved.
The numbers you need before you start
Four figures, all from documentation rather than memory:
- The minimum supply voltage. Not the nominal panel output and not what your meter reads on a good mains day. The relevant figure is the lowest the supply will fall to in the worst credible condition — running on standby batteries towards the end of the designed standby period.
- The alarm current of each device, from the manufacturer's data. Note whether the published figure is average or peak.
- The cable's resistance per metre for the conductor size actually installed, from the cable manufacturer's data. Remember the current travels out and back, so both conductors count.
- The minimum operating voltage of the device at the far end, from its data sheet.
If any of those four are guesses, the answer is a guess.
Doing the calculation
Work along the circuit in sections. For each section, multiply the current flowing in that section by the loop resistance of that section — that is, the resistance of the run out and the return. Then subtract the accumulated drop from the starting voltage.
The current in each section is the total alarm current of every device downstream of it, so the section nearest the panel carries the whole circuit load and the final section carries only the last device. This is why simply multiplying total current by total cable length overstates the drop, sometimes considerably, and why a section-by-section walk is worth doing when a circuit is marginal.
Compare the voltage arriving at the last device against its minimum operating voltage. If there is no headroom, the circuit needs changing before it is signed off — not a note in the report saying it worked on the day.
Proving it on site
Calculation tells you what should happen. Measurement tells you what is happening.
- Measure at the far end, in alarm, with the whole circuit operating. A reading taken with one device sounding tells you very little.
- Measure on standby power as well as mains where it is safe and practicable to do so, because that is the worst case.
- Record the panel output voltage at the same moment as the far-end reading. A drop is a difference between two figures; one figure on its own is not evidence.
- Check the terminations you pass on the way. A single loose or corroded connection in a junction box can add more resistance than a hundred metres of cable, and it will not appear in any calculation.
Common causes of unexpected drop
- Cable size reduced part-way along the run, usually where a circuit was extended by someone working with what was on the van.
- Devices added after commissioning without the circuit being reassessed.
- Poor terminations — loose screws, conductors trapped on insulation, corrosion in damp cabinets and external enclosures.
- Shared or borrowed supplies where an ancillary load has been picked up from a sounder circuit.
- Peak versus average current confusion, particularly with beacons and combined sounder-beacons.
- The wrong minimum supply voltage used in the calculation, giving false headroom.
Where the fix belongs
If the numbers do not work, there are four honest options: increase the conductor size, shorten the run, split the load over additional circuits, or install a local power supply closer to the far end. Each has a consequence. A local supply cures the drop but introduces another standby battery, another capacity calculation and another maintenance item, so it should be a design decision recorded in the documentation rather than a field improvisation.
What to record
- The circuit reference and what is on it.
- Device counts and the alarm current figure used for each type, with its source.
- Conductor size, cable type and measured or scaled run length.
- The minimum supply voltage assumed, and why.
- Calculated far-end voltage and the device's minimum operating voltage.
- Measured panel and far-end voltages, with the conditions under which they were taken.
The Regulatory Reform (Fire Safety) Order 2005 places the duty to keep fire safety measures in efficient working order on the responsible person. Evidence that a circuit was assessed properly — and under what assumptions — is part of how that duty is demonstrated when the system is later extended or investigated.
Relevant standards
BS 5839-1 is the code of practice for the design, installation, commissioning and maintenance of these systems in non-domestic premises. The BS EN 54 series covers the products themselves, including the operating voltage ranges that the calculation is checked against. Specific limits, cable requirements and supply arrangements must be taken from the current editions and from the manufacturer's documentation, not from general guidance such as this page.
Safety warning
Far-end voltage readings in alarm are taken on live circuits. Agree and record the isolation and the alarm test with the responsible person before you start, make sure nobody treats the sounders as a real evacuation, and restore the system to normal operation before leaving site. If a circuit has to be left disabled while a volt-drop problem is resolved, record the disablement and who was informed.
When not to rely on a calculation alone
A clean calculation on an as-designed drawing says nothing about the installation actually in the building. On an existing system, always confirm what is really connected before trusting the arithmetic, and treat a marginal result as a reason to measure rather than a reason to relax.
Related documentation
This sits naturally alongside the guidance on fire alarm cabling, on power supplies and standby batteries, and on diagnosing loop open and short circuit faults, where similar measurement discipline applies.
Professional disclaimer
This page is engineering support only. It is not a substitute for competent design, the manufacturer's instructions, the applicable standards or the responsible person's procedures. Work on live fire alarm systems should only be carried out by competent persons, with the appropriate isolations agreed and recorded.