# UPS vs Inverter: What Happens to Your Fibre Signal?

Source: https://urbanx.co.za/knowledge-hub/fno-troubleshooting/ups-vs-inverter-fibre-signal

**Answer:** A UPS or inverter does not affect the fibre signal itself, but unstable output can cause your ONT to reboot or fail to register on the PON network. If the ONT loses stable power, it restarts and re-enters line sync, which can temporarily interrupt connectivity.

Part of [FNO Troubleshooting & Fibre Diagnostics](https://urbanx.co.za/knowledge-hub/fno-troubleshooting) · 6 min read · by UrbanX Network Operations

## What the Problem Means

During loadshedding, many South African households rely on:

- UPS (Uninterruptible Power Supply)
- Inverter systems

These devices power:

- ONT
- Router
- Sometimes fibre wall unit

Fibre carries optical light and is not electrically damaged by power cuts. However, unstable backup power can cause:

- Repeated ONT reboots
- Extended PON flashing
- Temporary disconnections

> **INFO:** The fibre signal remains intact — the ONT loses registration due to power instability.

## How ONT Power Stability Affects Registration

When ONT voltage fluctuates:

- Device shuts down unexpectedly
- Optical session drops
- ONT restarts
- PON re-registration begins

If voltage dips repeatedly, the ONT may:

- Fail to complete line sync
- Show prolonged flashing PON
- Trigger temporary LOS state

> **WARNING:** This is not a fibre break. It is power instability at the device level.

## UPS vs Inverter Comparison (ONT Stability Focus)

| Feature | UPS | Inverter |
| --- | --- | --- |
| Switchover time | Near-instant | Slight delay possible |
| Voltage regulation | Usually stable | Depends on model |
| Risk of ONT reboot | Low | Moderate if unstable |
| Best for ONT stability | Yes | Yes (if pure sine wave) |
| Affects fibre cable? | No | No |

Important distinction: Backup power quality affects the ONT, not the fibre infrastructure.

## Step-by-Step Diagnostic Checklist

1. **Check ONT Power Light**
   - If flickering → unstable voltage.
2. **Observe PON After Switchover**
   - Flashing briefly → normal.
   - Flashing >10 minutes → possible restart loop.
3. **Avoid Rapid Manual Restart**
   - Allow 5 full minutes for sync.
4. **Test Direct Wall Power (When Grid Returns)**
   - If connection stabilizes immediately, backup system may be unstable.
5. **Check UPS/Inverter Output Rating**
   - ONT typically requires stable 12V DC output via adapter.

## Definition: Power Stability and Line Sync

- **Power Stability** — Consistent voltage delivery to the ONT power adapter.
- **Line Sync** — The registration handshake between ONT and the passive optical network (PON).

> **INFO:** If power fluctuates, line sync resets repeatedly, interrupting connectivity.

## South African Context: Extended Loadshedding

During prolonged outages:

- Fibre street cabinets may rely on battery backup
- Cabinet batteries may deplete after several hours
- Area-level outages can occur

If neighbours also lose connectivity, it may not be your UPS — it may be cabinet power depletion.

## When to Escalate

Escalate when:

- ONT has stable power
- PON does not stabilize
- LOS remains red after proper restart
- Neighbours also affected

Typical repair timelines in South Africa:

- 24–48 hours for isolated faults
- Up to 72 hours for infrastructure-level repair

Do not escalate if issue resolves immediately when grid power returns and backup power is removed.

## When This Is NOT the Issue

If all of the following are true, this is not your fault domain:
- PON is solid green
- LOS is off
- Internet is slow or unstable

Then the issue is not related to backup power stability.

## Frequently Asked Questions

**Can a UPS damage my fibre line?**

No. Fibre transmits light and is not electrically damaged by backup systems.

**Why does my internet drop when switching to inverter?**

Voltage dip during switchover may reboot the ONT.

**Is a pure sine wave inverter better for ONT stability?**

Yes. It delivers cleaner voltage similar to grid power.

**Should I power only the router or also the ONT?**

Both must remain powered to maintain connectivity.

**Why do I still lose internet during long outages?**

Street-level fibre cabinets may lose backup power after extended loadshedding.