---
title: I.S. Parameters
description: "## Main power supply voltage (Um) **Um** (max. rms AC or DC voltage) refers to the **maximum voltage** that can be **applied to the non-intrinsically safe terminals** of intrinsically safe unit (supply side of associated apparatus). The permitted **value of…"
url: "https://www.eicstech.com/kb/intrinsic-safety-ex-i/basics-on-intrinsically-safe-parameter/"
updated: "2022-06-01"
category: Explosion Proof Basics / Intrinsic Safety (Ex i)
---

# I.S. Parameters

## Main power supply voltage (Um)

**Um** (max. rms AC or DC voltage) refers to the **maximum voltage** that can be **applied to the non-intrinsically safe terminals** of intrinsically safe unit (supply side of associated apparatus). The permitted **value of Um** is **given** in the **documentation** and is usually marked on the associated apparatus label.

The documentation will also show the maximum **output voltage** and **current** that can be taken, and the allowable **inductance**and **capacitance** for the circuit to be connected to the interface (**U~o, I~o, L~a & C~a).**

The documentation (and possibly the labelling) of the field equipment will give values for maximum **input voltage** and **current** that can be applied on the connection facilities. The **inductance** and **capacitance** of the **equipment** will be given and all this data will be considered when specifying the associated apparatus (interface) and the **cable** requirements **(U~i, I~i, L~i& C~i).**

## Equipment that can be connected to I.S. interfaces

**Only the apparatus specified in the system documentation may be connected to the output of the interface (safety barrier).**

In addition to cabling conforming to the system requirements, the documentation will include certified intrinsically safe field equipment and simple apparatus.

## Analysis of Intrinsically Safe Systems and its parameters

The **safety barrier** label and/or documentation will indicate the maximum voltage and current that it can **deliver** under fault conditions, and the maximum permitted inductance (L) and capacitance ( C) that may be connected to its hazardous area terminals.

The **field device** label and/or its documentation will indicates he maximum voltage and current it can **receive**, and the values of its own inductance and capacitance.

The electrical voltage and current parameters must be matched and the capacitance and inductance should be verified that they are within the safety limits.

The field device may then be **used safely** with any barrier whose maximum **voltage and current** are **equal** **to** **or less than the field device** values.

The table shows the required relationship between the parameters.

| **Safety Barrier parameters** | ** ** | **Field equipment (FE) parameters** | ** ** |
| --- | --- | --- | --- |
| Voc = Uo | Voltage | Ui = Vmax | Volatge |
| Isc = Io | Current | Ii = Imax | Curent |
| Ca | Capacitance | Ci | Capacitance |
| La | Inductance | Li | Inductance |

**Voc ≤ Vmax                         Barrier output voltage not greater than FE              max input voltage**

**Isc ≤ Imax                            Barrier output current not greater than FE               max input current**

**Ca > Ci + C cable                 Barrier allowable connected C more than                 FE C + cable C**

**La > Li + L cable                  Barrier allowable connected L more than                  FE C + cable L**

Example:

![example of apparatus with parameters](https://www.eicstech.com/wp-content/uploads/2022/06/example-of-apparatus-with-parameters.png)